Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of investment securities
Amortized Cost	$ 2,862,764	$ 3,208,941
Gross Unrealized Gains	86,393	49,801
Gross Unrealized Losses	(10,783)	(35,913)
Fair Value	2,938,374	3,222,829
Available-for-sale securities
Summary of investment securities
Amortized Cost	2,341,767	3,208,941
Gross Unrealized Gains	53,468	49,801
Gross Unrealized Losses	(7,807)	(35,913)
Fair Value	2,387,428	3,222,829
Available-for-sale securities | U.S. government and federal agency
Summary of investment securities
Amortized Cost	44
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	44
Available-for-sale securities | U.S. government sponsored enterprises
Summary of investment securities
Amortized Cost	21,916	10,441
Gross Unrealized Gains	31	187
Gross Unrealized Losses	(2)	0
Fair Value	21,945	10,628
Available-for-sale securities | State and local governments
Summary of investment securities
Amortized Cost	962,365	1,377,347
Gross Unrealized Gains	40,173	31,621
Gross Unrealized Losses	(4,569)	(23,890)
Fair Value	997,969	1,385,078
Available-for-sale securities | Corporate bonds
Summary of investment securities
Amortized Cost	313,545	440,337
Gross Unrealized Gains	2,059	3,922
Gross Unrealized Losses	(750)	(1,758)
Fair Value	314,854	442,501
Available-for-sale securities | Residential mortgage-backed securities
Summary of investment securities
Amortized Cost	1,043,897	1,380,816
Gross Unrealized Gains	11,205	14,071
Gross Unrealized Losses	(2,486)	(10,265)
Fair Value	1,052,616	1,384,622
Held-to-maturity securities
Summary of investment securities
Amortized Cost	520,997
Gross Unrealized Gains	32,925
Gross Unrealized Losses	(2,976)
Fair Value	550,946
Held-to-maturity securities | State and local governments
Summary of investment securities
Amortized Cost	520,997
Gross Unrealized Gains	32,925
Gross Unrealized Losses	(2,976)
Fair Value	$ 550,946