Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale
Amortized Cost	$ 2,336,693	$ 2,341,767
Gross Unrealized Gains	42,135	53,468
Gross Unrealized Losses	(16,998)	(7,807)
Fair Value	2,361,830	2,387,428
Held-to-maturity
Amortized Cost	593,314	520,997
Gross Unrealized Gains	23,308	32,925
Gross Unrealized Losses	(7,841)	(2,976)
Fair Value	608,781	550,946
Total
Amortized Cost	2,930,007	2,862,764
Gross Unrealized Gains	65,443	86,393
Gross Unrealized Losses	(24,839)	(10,783)
Fair Value	2,970,611	2,938,374
U.S. government and federal agency
Available-for-sale
Amortized Cost	51,889	44
Gross Unrealized Gains	13	0
Gross Unrealized Losses	(405)	0
Fair Value	51,497	44
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	81,085	21,916
Gross Unrealized Gains	32	31
Gross Unrealized Losses	(356)	(2)
Fair Value	80,761	21,945
State and local governments
Available-for-sale
Amortized Cost	938,088	962,365
Gross Unrealized Gains	29,310	40,173
Gross Unrealized Losses	(9,346)	(4,569)
Fair Value	958,052	997,969
Held-to-maturity
Amortized Cost	593,314	520,997
Gross Unrealized Gains	23,308	32,925
Gross Unrealized Losses	(7,841)	(2,976)
Fair Value	608,781	550,946
Corporate bonds
Available-for-sale
Amortized Cost	355,140	313,545
Gross Unrealized Gains	1,482	2,059
Gross Unrealized Losses	(773)	(750)
Fair Value	355,849	314,854
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	910,491	1,043,897
Gross Unrealized Gains	11,298	11,205
Gross Unrealized Losses	(6,118)	(2,486)
Fair Value	$ 915,671	$ 1,052,616