Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale:
Amortized Cost	$ 2,229,300	$ 1,625,000
Gross Unrealized Gains	3,900	6,700
Gross Unrealized Losses	(24,500)	(20,000)
Estimated Fair Value	2,208,712	1,611,698
Held-to-Maturity:
Amortized Cost	484,494	512,770
Gross Unrealized Gains	11,500	10,700
Gross Unrealized Losses	(12,700)	(10,100)
Estimated Fair Value	483,349	513,273
US Treasury notes
Available-for-Sale:
Amortized Cost	3,200	3,600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,200	3,600
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	172,400	160,200
Gross Unrealized Gains	2,600	2,700
Gross Unrealized Losses	(600)	(500)
Estimated Fair Value	174,400	162,400
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	569,500	397,400
Gross Unrealized Gains	0	300
Gross Unrealized Losses	(8,000)	(6,400)
Estimated Fair Value	561,500	391,300
Held-to-Maturity:
Amortized Cost	19,800	19,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(200)	(200)
Estimated Fair Value	19,600	19,600
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,474,100	1,220,900
Gross Unrealized Gains	3,800	6,400
Gross Unrealized Losses	(15,400)	(13,600)
Estimated Fair Value	1,462,500	1,213,700
Held-to-Maturity:
Amortized Cost	230,500	279,600
Gross Unrealized Gains	8,800	7,800
Gross Unrealized Losses	(11,600)	(9,200)
Estimated Fair Value	227,700	278,200
Corporate securities
Available-for-Sale:
Amortized Cost	88,000
Gross Unrealized Gains	100
Gross Unrealized Losses	(300)
Estimated Fair Value	87,800
Held-to-Maturity:
Amortized Cost	61,600	53,000
Gross Unrealized Gains	100	200
Gross Unrealized Losses	(300)	(200)
Estimated Fair Value	61,400	53,000
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	91,500	100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(800)	0
Estimated Fair Value	90,700	100
Other Investments
Available-for-Sale:
Amortized Cost	3,000	3,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,000	3,000
Other Investments
Held-to-Maturity:
Amortized Cost	200	200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 200	$ 200