Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 2,207,876
Corporate debt securities	2,219,194
Corporate Debt Securities Issued by Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	95,558	97,109
Corporate debt securities	94,120	96,127
Single-issuer Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	47,590	47,569
Corporate debt securities	42,863	42,016
Subordinated Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	47,563	47,530
Corporate debt securities	50,173	50,023
Pooled Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	405	2,010
Corporate debt securities	1,084	4,088
Other Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,562	1,907
Corporate debt securities	3,562	1,907
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	99,120	99,016
Corporate debt securities	$ 97,682	$ 98,034