Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale
Amortized Cost	$ 2,119,194	$ 2,422,802
Gross Unrealized Gains	30,598	24,960
Gross Unrealized Losses	(7,320)	(22,285)
Fair Value	2,142,472	2,425,477
Held-to-maturity
Amortized Cost	659,347	675,674
Gross Unrealized Gains	26,045	21,400
Gross Unrealized Losses	(6,897)	(7,985)
Fair Value	678,495	689,089
Total
Amortized Cost	2,778,541	3,098,476
Gross Unrealized Gains	56,643	46,360
Gross Unrealized Losses	(14,217)	(30,270)
Fair Value	2,820,967	3,114,566
U.S. government and federal agency
Available-for-sale
Amortized Cost	35,774	39,554
Gross Unrealized Gains	30	15
Gross Unrealized Losses	(193)	(162)
Fair Value	35,611	39,407
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	19,161	19,557
Gross Unrealized Gains	122	55
Gross Unrealized Losses	(9)	(42)
Fair Value	19,274	19,570
State and local governments
Available-for-sale
Amortized Cost	646,916	775,395
Gross Unrealized Gains	24,580	20,941
Gross Unrealized Losses	(3,593)	(9,963)
Fair Value	667,903	786,373
Held-to-maturity
Amortized Cost	659,347	675,674
Gross Unrealized Gains	26,045	21,400
Gross Unrealized Losses	(6,897)	(7,985)
Fair Value	678,495	689,089
Corporate bonds
Available-for-sale
Amortized Cost	421,208	471,569
Gross Unrealized Gains	1,423	1,175
Gross Unrealized Losses	(262)	(793)
Fair Value	422,369	471,951
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	894,877	1,014,518
Gross Unrealized Gains	4,265	2,744
Gross Unrealized Losses	(2,778)	(9,747)
Fair Value	896,364	1,007,515
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	101,258	102,209
Gross Unrealized Gains	178	30
Gross Unrealized Losses	(485)	(1,578)
Fair Value	$ 100,951	$ 100,661