Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Summary of investment securities
Amortized Cost	$ 3,122,092		$ 3,208,941
Gross Unrealized Gains	68,071		49,801
Gross Unrealized Losses	(22,081)		(35,913)
Fair Value	3,168,082		3,222,829
Available-for-sale securities
Summary of investment securities
Amortized Cost	2,640,616		3,208,941
Gross Unrealized Gains	44,566		49,801
Gross Unrealized Losses	(16,002)		(35,913)
Fair Value	2,669,180		3,222,829
Available-for-sale securities | U.S. government sponsored enterprises
Summary of investment securities
Amortized Cost	9,792		10,441
Gross Unrealized Gains	150		187
Gross Unrealized Losses	0		0
Fair Value	9,942		10,628
Available-for-sale securities | State and local governments
Summary of investment securities
Amortized Cost	902,464		1,377,347
Gross Unrealized Gains	27,337		31,621
Gross Unrealized Losses	(8,351)		(23,890)
Fair Value	921,450		1,385,078
Available-for-sale securities | Corporate bonds
Summary of investment securities
Amortized Cost	431,831		440,337
Gross Unrealized Gains	4,567		3,922
Gross Unrealized Losses	(713)		(1,758)
Fair Value	435,685		442,501
Available-for-sale securities | Residential mortgage-backed securities
Summary of investment securities
Amortized Cost	1,296,529	[1]	1,380,816	[1]
Gross Unrealized Gains	12,512	[1]	14,071	[1]
Gross Unrealized Losses	(6,938)	[1]	(10,265)	[1]
Fair Value	1,302,103	[1]	1,384,622	[1]
Held-to-maturity securities
Summary of investment securities
Amortized Cost	481,476
Gross Unrealized Gains	23,505
Gross Unrealized Losses	(6,079)
Fair Value	498,902
Held-to-maturity securities | State and local governments
Summary of investment securities
Amortized Cost	481,476
Gross Unrealized Gains	23,505
Gross Unrealized Losses	(6,079)
Fair Value	$ 498,902

[1] Residential mortgage-backed securities are obligations of U.S. government sponsored enterprises and include collateralized mortgage obligations.