Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale:
Amortized Cost	$ 2,325,000	$ 2,229,300
Gross Unrealized Gains	2,000	3,900
Gross Unrealized Losses	(46,300)	(24,500)
Estimated Fair Value	2,280,700	2,208,700
Held-to-Maturity:
Amortized Cost	462,300	484,500
Gross Unrealized Gains	9,600	11,500
Gross Unrealized Losses	(16,400)	(12,700)
Estimated Fair Value	455,500	483,300
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	3,200	3,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,200	3,200
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	562,800	569,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13,000)	(8,000)
Estimated Fair Value	549,800	561,500
Held-to-Maturity:
Amortized Cost	19,800	19,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(300)	(200)
Estimated Fair Value	19,500	19,600
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,578,200	1,474,100
Gross Unrealized Gains	1,900	3,800
Gross Unrealized Losses	(30,800)	(15,400)
Estimated Fair Value	1,549,300	1,462,500
Held-to-Maturity:
Amortized Cost	217,600	230,500
Gross Unrealized Gains	7,700	8,800
Gross Unrealized Losses	(14,100)	(11,600)
Estimated Fair Value	211,200	227,700
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	84,700	91,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,500)	(800)
Estimated Fair Value	83,200	90,700
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	168,300	172,400
Gross Unrealized Gains	1,800	2,600
Gross Unrealized Losses	(1,300)	(600)
Estimated Fair Value	168,800	174,400
Corporate securities
Available-for-Sale:
Amortized Cost	93,100	88,000
Gross Unrealized Gains	100	100
Gross Unrealized Losses	(1,000)	(300)
Estimated Fair Value	92,200	87,800
Held-to-Maturity:
Amortized Cost	56,500	61,600
Gross Unrealized Gains	100	100
Gross Unrealized Losses	(700)	(300)
Estimated Fair Value	55,900	61,400
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	100	200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 100	$ 200