Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 959,025
Available-for-sale Securities, Due after one year through five years, Amortized Cost	94,816
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	229,839
Available-for-sale Securities, Due after ten years, Amortized Cost	747,184
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	959,287
Available-for-sale Securities, Due after one year through five years, Fair Value	95,302
Available-for-sale Securities, Due after five years through ten years, Fair Value	230,299
Available-for-sale Securities, Due after ten years, Fair Value	746,741
Available-for-sale securities	$ 2,031,629	$ 1,938,499
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.27%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	1.17%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	0.90%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	0.86%
Available-for-sale Securities, Total Weighted-Average Yield	0.60%