Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale
Amortized Cost	$ 2,568,341	$ 2,588,013
Gross Unrealized Gains	46,583	41,065
Gross Unrealized Losses	(10,299)	(18,318)
Fair Value	2,604,625	2,610,760
Held-to-maturity
Amortized Cost	691,663	702,072
Gross Unrealized Gains	33,081	31,863
Gross Unrealized Losses	(4,554)	(4,422)
Fair Value	720,190	729,513
Total
Amortized Cost	3,260,004	3,290,085
Gross Unrealized Gains	79,664	72,928
Gross Unrealized Losses	(14,853)	(22,740)
Fair Value	3,324,815	3,340,273
U.S. government and federal agency
Available-for-sale
Amortized Cost	45,957	47,868
Gross Unrealized Gains	18	15
Gross Unrealized Losses	(288)	(432)
Fair Value	45,687	47,451
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	68,100	93,230
Gross Unrealized Gains	673	100
Gross Unrealized Losses	0	(163)
Fair Value	68,773	93,167
State and local governments
Available-for-sale
Amortized Cost	870,239	856,738
Gross Unrealized Gains	36,720	34,159
Gross Unrealized Losses	(5,826)	(5,878)
Fair Value	901,133	885,019
Held-to-maturity
Amortized Cost	691,663	702,072
Gross Unrealized Gains	33,081	31,863
Gross Unrealized Losses	(4,554)	(4,422)
Fair Value	720,190	729,513
Corporate bonds
Available-for-sale
Amortized Cost	443,632	386,629
Gross Unrealized Gains	738	611
Gross Unrealized Losses	(1,495)	(3,077)
Fair Value	442,875	384,163
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	1,140,413	1,203,548
Gross Unrealized Gains	8,434	6,180
Gross Unrealized Losses	(2,690)	(8,768)
Fair Value	$ 1,146,157	$ 1,200,960