Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale
Amortized Cost	$ 2,494,301	$ 2,341,767
Gross Unrealized Gains	47,457	53,468
Gross Unrealized Losses	(10,764)	(7,807)
Fair Value	2,530,994	2,387,428
Held-to-maturity
Amortized Cost	651,822	520,997
Gross Unrealized Gains	26,470	32,925
Gross Unrealized Losses	(5,897)	(2,976)
Fair Value	672,395	550,946
Total
Amortized Cost	3,146,123	2,862,764
Gross Unrealized Gains	73,927	86,393
Gross Unrealized Losses	(16,661)	(10,783)
Fair Value	3,203,389	2,938,374
U.S. government and federal agency
Available-for-sale
Amortized Cost	49,712	44
Gross Unrealized Gains	15	0
Gross Unrealized Losses	(412)	0
Fair Value	49,315	44
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	98,095	21,916
Gross Unrealized Gains	699	31
Gross Unrealized Losses	(3)	(2)
Fair Value	98,791	21,945
State and local governments
Available-for-sale
Amortized Cost	903,722	962,365
Gross Unrealized Gains	34,084	40,173
Gross Unrealized Losses	(6,806)	(4,569)
Fair Value	931,000	997,969
Held-to-maturity
Amortized Cost	651,822	520,997
Gross Unrealized Gains	26,470	32,925
Gross Unrealized Losses	(5,897)	(2,976)
Fair Value	672,395	550,946
Corporate bonds
Available-for-sale
Amortized Cost	360,846	313,545
Gross Unrealized Gains	1,219	2,059
Gross Unrealized Losses	(1,254)	(750)
Fair Value	360,811	314,854
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	1,081,926	1,043,897
Gross Unrealized Gains	11,440	11,205
Gross Unrealized Losses	(2,289)	(2,486)
Fair Value	$ 1,091,077	$ 1,052,616