Investment Securities - Debt Maturities (Details) - Investment Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Due within one year, Amortized Cost	$ 1,256,154
Available-for-sale Securities, Due after one year through five years, Amortized Cost	529,229
Available-for-sale Securities, Due after five years through ten years, Amortized Cost	393,580
Available-for-sale Securities, Due after ten years, Amortized Cost	825,270
Available-for-sale Securities, Amortized Cost Basis	3,004,233	$ 2,779,047
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Fair Value	1,256,654
Available-for-sale Securities, Due after one year through five years, Fair Value	529,322
Available-for-sale Securities, Due after five years through ten years, Fair Value	393,905
Available-for-sale Securities, Due after ten years, Fair Value	821,304
Available-for-sale securities	$ 3,001,185	$ 2,775,025
Available-for-sale Securities, Debt Maturities, Weighted Average Yield, Single Maturity Date [Abstract]
Available-for-sale Securities, Due within one year, Weighted-Average Yield	0.61%
Available-for-sale Securities, Due after one year through five years, Weighted-Average Yield	0.99%
Available-for-sale Securities, Due after five years through ten years, Weighted-Average Yield	1.18%
Available-for-sale Securities, Due after ten years, Weighted-Average Yield	1.11%
Available-for-sale Securities, Total Weighted-Average Yield	0.89%