Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Sep. 30, 2016 USD ($)
Amortized Cost
Due in one year or less	$ 50,521
Due from one year to five years	34,278
Due from five years to ten years	96,172
Due after ten years	429,930
Amortized cost, before securities without debt maturities	610,901
Amortized Cost	2,463,732
Estimated Fair Value
Due in one year or less	51,020
Due from one year to five years	35,356
Due from five years to ten years	99,187
Due after ten years	421,116
Available for sale securities, debt maturities, before securities without single maturities	606,679
Estimated Fair Value	2,485,585
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	651,060
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	652,359
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,201,771
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,226,547