Investment Securities - Debt Maturities (Details) (Investment Securities [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investment Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,250,632
Available-for-sale Securities, Due after one year through five years, Amortized Cost	177,581
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	303,144
Available-for-sale Securities, Due after ten years, Amortized Cost	584,684
Available-for-sale Securities, Amortized Cost Basis	2,316,041	2,485,421
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,250,753
Available-for-sale Securities, Due after one year through five years, Fair Value	178,519
Available-for-sale Securities, Due after five years through ten years, Fair Value	301,413
Available-for-sale Securities, Due after ten years, Fair Value	585,278
Available-for-sale securities	$ 2,315,963	$ 2,483,147
Available-for-sale Securities, Debt Maturities, Weighted-Average Yield, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.32%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	0.90%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	0.79%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	2.35%
Available-for-sale Securities, Total Weighted-Average Yield	0.94%