Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-Sale:
Amortized Cost	$ 2,226,200	$ 2,229,300
Gross Unrealized Gains	1,700	3,900
Gross Unrealized Losses	(49,900)	(24,500)
Estimated Fair Value	2,178,000	2,208,700
Held-to-Maturity:
Amortized Cost	440,900	484,500
Gross Unrealized Gains	9,400	11,500
Gross Unrealized Losses	(17,500)	(12,700)
Estimated Fair Value	432,800	483,300
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	1,900	3,200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1,900	3,200
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	557,000	569,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13,800)	(8,000)
Estimated Fair Value	543,200	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	1,491,700	1,474,100
Gross Unrealized Gains	1,600	3,800
Gross Unrealized Losses	(33,400)	(15,400)
Estimated Fair Value	1,459,900	1,462,500
Held-to-Maturity:
Amortized Cost	207,600	230,500
Gross Unrealized Gains	7,700	8,800
Gross Unrealized Losses	(15,200)	(11,600)
Estimated Fair Value	200,100	227,700
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	80,100	91,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,700)	(800)
Estimated Fair Value	78,400	90,700
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	162,100	172,400
Gross Unrealized Gains	1,600	2,600
Gross Unrealized Losses	(1,300)	(600)
Estimated Fair Value	162,400	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	51,400	61,600
Gross Unrealized Gains	100	100
Gross Unrealized Losses	(700)	(300)
Estimated Fair Value	50,800	61,400
Other investments
Available-for-Sale:
Amortized Cost	2,500	3,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	2,500	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