Fair Value Measurements on a Recurring Basis (Details) (Recurring Measurements, USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Financial Assets
Total assets measured at fair value on a recurring basis	$ 3,318,953	$ 3,683,005
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	2,004	16,832
Interest rate swaps
Financial Liabilities
Interest rate swaps	2,004	16,832
U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale		202
U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	11,556	17,480
State and local governments
Financial Assets
Investment securities, available-for-sale	1,341,384	1,214,518
Corporate bonds
Financial Assets
Investment securities, available-for-sale	442,845	288,795
Collateralized debt obligations
Financial Assets
Investment securities, available-for-sale		1,708
Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,523,168	2,160,302
Level 1
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 1 | Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 1 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale		0
Level 1 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Collateralized debt obligations
Financial Assets
Investment securities, available-for-sale		0
Level 1 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 2
Financial Assets
Total assets measured at fair value on a recurring basis	3,318,953	3,683,005
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	2,004	16,832
Level 2 | Interest rate swaps
Financial Liabilities
Interest rate swaps	2,004	16,832
Level 2 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale		202
Level 2 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	11,556	17,480
Level 2 | State and local governments
Financial Assets
Investment securities, available-for-sale	1,341,384	1,214,518
Level 2 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	442,845	288,795
Level 2 | Collateralized debt obligations
Financial Assets
Investment securities, available-for-sale		1,708
Level 2 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,523,168	2,160,302
Level 3
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 3 | Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 3 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale		0
Level 3 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Collateralized debt obligations
Financial Assets
Investment securities, available-for-sale		0
Level 3 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	$ 0	$ 0