How did the Robertson family make their money?

Bank of Monroe 3 Baxter State Bank 3 Connection Bank 12 Dedicated Community Bank 8 First State Bank of Texas 3 FIRST WESTERN Trust Bank 5 Gateway Commercial Bank 4 Independence Bank 2 KITSAP Bank 18 Montezuma State Bank 4 Nantahala Bank & Trust Company 6 Ohio State Bank 1 Perryton National Bank 10 Premier Bank of the South 4 Range Bank 6 Security State Bank of Marine 2 Southern Heritage Bank 2 State Bank of Lakota 4 Sterling State Bank 18 Sunrise Bank 14