Fair Value Measurements on a Recurring Basis (Details) (Recurring Measurements, USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financial Assets
Total assets measured at fair value on a recurring basis	$ 2,669,180	$ 3,224,725
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	3,583
Interest rate swaps
Financial Liabilities
Interest rate swaps	3,583
U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	9,942	10,628
State and local governments
Financial Assets
Investment securities, available-for-sale	921,450	1,385,078
Corporate bonds
Financial Assets
Investment securities, available-for-sale	435,685	442,501
Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,302,103	1,384,622
Interest rate swaps
Financial Assets
Interest rate swaps		1,896
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
Level 1 | Interest rate swaps
Financial Liabilities
Interest rate swaps	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 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Interest rate swaps
Financial Assets
Interest rate swaps		0
Level 2
Financial Assets
Total assets measured at fair value on a recurring basis	2,669,180	3,224,725
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	3,583
Level 2 | Interest rate swaps
Financial Liabilities
Interest rate swaps	3,583
Level 2 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	9,942	10,628
Level 2 | State and local governments
Financial Assets
Investment securities, available-for-sale	921,450	1,385,078
Level 2 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	435,685	442,501
Level 2 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	1,302,103	1,384,622
Level 2 | Interest rate swaps
Financial Assets
Interest rate swaps		1,896
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
Level 3 | Interest rate swaps
Financial Liabilities
Interest rate swaps	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 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Interest rate swaps
Financial Assets
Interest rate swaps		$ 0