Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale
Amortized Cost	$ 2,588,013	$ 2,341,767
Gross Unrealized Gains	41,065	53,468
Gross Unrealized Losses	(18,318)	(7,807)
Fair Value	2,610,760	2,387,428
Held-to-maturity
Amortized Cost	702,072	520,997
Gross Unrealized Gains	31,863	32,925
Gross Unrealized Losses	(4,422)	(2,976)
Fair Value	729,513	550,946
Total
Amortized Cost	3,290,085	2,862,764
Gross Unrealized Gains	72,928	86,393
Gross Unrealized Losses	(22,740)	(10,783)
Fair Value	3,340,273	2,938,374
U.S. government and federal agency
Available-for-sale
Amortized Cost	47,868	44
Gross Unrealized Gains	15	0
Gross Unrealized Losses	(432)	0
Fair Value	47,451	44
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	93,230	21,916
Gross Unrealized Gains	100	31
Gross Unrealized Losses	(163)	(2)
Fair Value	93,167	21,945
State and local governments
Available-for-sale
Amortized Cost	856,738	962,365
Gross Unrealized Gains	34,159	40,173
Gross Unrealized Losses	(5,878)	(4,569)
Fair Value	885,019	997,969
Held-to-maturity
Amortized Cost	702,072	520,997
Gross Unrealized Gains	31,863	32,925
Gross Unrealized Losses	(4,422)	(2,976)
Fair Value	729,513	550,946
Corporate bonds
Available-for-sale
Amortized Cost	386,629	313,545
Gross Unrealized Gains	611	2,059
Gross Unrealized Losses	(3,077)	(750)
Fair Value	384,163	314,854
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	1,203,548	1,043,897
Gross Unrealized Gains	6,180	11,205
Gross Unrealized Losses	(8,768)	(2,486)
Fair Value	$ 1,200,960	$ 1,052,616