Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-Sale:
Amortized Cost	$ 2,213,200	$ 2,229,300
Gross Unrealized Gains	1,500	3,900
Gross Unrealized Losses	(55,900)	(24,500)
Estimated Fair Value	2,158,800	2,208,700
Held-to-Maturity:
Amortized Cost	417,800	484,500
Gross Unrealized Gains	9,000	11,500
Gross Unrealized Losses	(19,400)	(12,700)
Estimated Fair Value	407,400	483,300
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	3,000	3,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,000	3,200
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	576,200	569,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15,300)	(8,000)
Estimated Fair Value	560,900	561,500
Held-to-Maturity:
Amortized Cost	19,700	19,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(300)	(200)
Estimated Fair Value	19,400	19,600
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,462,000	1,474,100
Gross Unrealized Gains	1,400	3,800
Gross Unrealized Losses	(37,800)	(15,400)
Estimated Fair Value	1,425,600	1,462,500
Held-to-Maturity:
Amortized Cost	198,000	230,500
Gross Unrealized Gains	7,800	8,800
Gross Unrealized Losses	(16,800)	(11,600)
Estimated Fair Value	189,000	227,700
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	74,800	91,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,800)	(800)
Estimated Fair Value	73,000	90,700
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	153,700	172,400
Gross Unrealized Gains	1,200	2,600
Gross Unrealized Losses	(1,700)	(600)
Estimated Fair Value	153,200	174,400
Corporate securities
Available-for-Sale:
Amortized Cost	93,000	88,000
Gross Unrealized Gains	100	100
Gross Unrealized Losses	(1,000)	(300)
Estimated Fair Value	92,100	87,800
Held-to-Maturity:
Amortized Cost	46,300	61,600
Gross Unrealized Gains	0	100
Gross Unrealized Losses	(600)	(300)
Estimated Fair Value	45,700	61,400
Other investments
Available-for-Sale:
Amortized Cost	4,200	3,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	4,200	3,000
Other Investments
Held-to-Maturity:
Amortized Cost	100	200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 100	$ 200