Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 317,502
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,089,290
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	70,534
Available-for-Sale Amortized Cost, After Ten Years	70,891
Available-for-Sale, Amortized Cost	1,548,217	$ 1,450,396
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	320,927
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,099,333
Available-for-Sale Estimated Fair Value, After Ten Years	71,242
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	71,848
Available-for-Sale, Estimated Fair Value	1,563,350	1,456,840
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	87,930
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	328,090
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	125,412
Held-to-Maturity Amortized Cost, After Ten Years	39,958
Held-to-Maturity, Amortized Cost	581,390	600,665
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	90,154
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	333,231
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	128,977
Held-to-Maturity Estimated Fair Value, After Ten Years	41,542
Held-to-Maturity, Estimated Fair Value	593,904	$ 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	277,491
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	277,837
Callable Structured Notes
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	10,000
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 10,000