Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 340,072
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,030,847
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	108,831
Available-for-Sale Amortized Cost, After Ten Years	32,033
Available-for-Sale, Amortized Cost	1,511,783	$ 1,450,396
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	345,636
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,045,108
Available-for-Sale Estimated Fair Value, After Ten Years	109,852
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	32,625
Available-for-Sale, Estimated Fair Value	1,533,221	1,456,840
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	99,864
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	290,601
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	117,829
Held-to-Maturity Amortized Cost, After Ten Years	30,758
Held-to-Maturity, Amortized Cost	539,052	600,665
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	97,362
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	304,804
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	121,191
Held-to-Maturity Estimated Fair Value, After Ten Years	30,691
Held-to-Maturity, Estimated Fair Value	554,048	$ 607,550
Callable Within One Year
Available-for-sale Securities and Held-to-maturity Securities
Investment Securities Primarily Classified as Available-for-Sale, Amortized Costs, After One Year but Within Five Years	220,907
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 221,189