Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale
Amortized Cost	$ 2,238,067	$ 2,588,013
Gross Unrealized Gains	60,394	41,065
Gross Unrealized Losses	(6,382)	(18,318)
Fair Value	2,292,079	2,610,760
Held-to-maturity
Amortized Cost	679,707	702,072
Gross Unrealized Gains	39,542	31,863
Gross Unrealized Losses	(4,354)	(4,422)
Fair Value	714,895	729,513
Total
Amortized Cost	2,917,774	3,290,085
Gross Unrealized Gains	99,936	72,928
Gross Unrealized Losses	(10,736)	(22,740)
Fair Value	3,006,974	3,340,273
U.S. government and federal agency
Available-for-sale
Amortized Cost	41,727	47,868
Gross Unrealized Gains	24	15
Gross Unrealized Losses	(158)	(432)
Fair Value	41,593	47,451
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	16,100	93,230
Gross Unrealized Gains	267	100
Gross Unrealized Losses	0	(163)
Fair Value	16,367	93,167
State and local governments
Available-for-sale
Amortized Cost	816,898	856,738
Gross Unrealized Gains	40,925	34,159
Gross Unrealized Losses	(5,764)	(5,878)
Fair Value	852,059	885,019
Held-to-maturity
Amortized Cost	679,707	702,072
Gross Unrealized Gains	39,542	31,863
Gross Unrealized Losses	(4,354)	(4,422)
Fair Value	714,895	729,513
Corporate bonds
Available-for-sale
Amortized Cost	401,291	386,629
Gross Unrealized Gains	3,154	611
Gross Unrealized Losses	(32)	(3,077)
Fair Value	404,413	384,163
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	962,051	1,203,548
Gross Unrealized Gains	16,024	6,180
Gross Unrealized Losses	(428)	(8,768)
Fair Value	$ 977,647	$ 1,200,960