Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Amortized Cost
Due in one year or less	$ 23,940
Due from one year to five years	30,708
Due from five years to ten years	114,114
Due after ten years	452,277
Amortized cost, before securities without debt maturities	621,039
Amortized Cost	2,569,641
Estimated Fair Value
Due in one year or less	24,118
Due from one year to five years	31,196
Due from five years to ten years	115,336
Due after ten years	440,411
Available for sale securities, debt maturities, before securities without single maturities	611,061
Estimated Fair Value	2,548,457
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	601,845
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	593,678
Commercial Mortgage Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	161,960
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	161,632
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	1,184,797
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	$ 1,182,086