Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale
Amortized Cost	$ 2,424,592	$ 2,588,013
Gross Unrealized Gains	69,662	41,065
Gross Unrealized Losses	(6,299)	(18,318)
Fair Value	2,487,955	2,610,760
Held-to-maturity
Amortized Cost	680,574	702,072
Gross Unrealized Gains	48,980	31,863
Gross Unrealized Losses	(3,189)	(4,422)
Fair Value	726,365	729,513
Total
Amortized Cost	3,105,166	3,290,085
Gross Unrealized Gains	118,642	72,928
Gross Unrealized Losses	(9,488)	(22,740)
Fair Value	3,214,320	3,340,273
U.S. government and federal agency
Available-for-sale
Amortized Cost	43,573	47,868
Gross Unrealized Gains	33	15
Gross Unrealized Losses	(220)	(432)
Fair Value	43,386	47,451
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	46,307	93,230
Gross Unrealized Gains	592	100
Gross Unrealized Losses	0	(163)
Fair Value	46,899	93,167
State and local governments
Available-for-sale
Amortized Cost	858,382	856,738
Gross Unrealized Gains	49,742	34,159
Gross Unrealized Losses	(5,335)	(5,878)
Fair Value	902,789	885,019
Held-to-maturity
Amortized Cost	680,574	702,072
Gross Unrealized Gains	48,980	31,863
Gross Unrealized Losses	(3,189)	(4,422)
Fair Value	726,365	729,513
Corporate bonds
Available-for-sale
Amortized Cost	434,209	386,629
Gross Unrealized Gains	3,076	611
Gross Unrealized Losses	(192)	(3,077)
Fair Value	437,093	384,163
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	1,042,121	1,203,548
Gross Unrealized Gains	16,219	6,180
Gross Unrealized Losses	(552)	(8,768)
Fair Value	$ 1,057,788	$ 1,200,960