Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 5,946
Due from one year to five years	52,613
Due from five years to ten years	105,838
Due after ten years	337,604
Amortized cost, before securities without debt maturities	502,001
Amortized Cost	2,071,734
Estimated Fair Value
Due in one year or less	5,949
Due from one year to five years	52,541
Due from five years to ten years	104,579
Due after ten years	324,585
Available for sale securities, debt maturities, before securities without single maturities	487,654
Estimated Fair Value	2,008,816
Amortized Cost
Due in one year or less	0
Due from one year to five years	0
Due from five years to ten years	0
Due after ten years	156,267
Debt securities, held-to-maturity, maturity, allocated and single maturity date, amortized cost, total	156,267
Amortized Cost, Held to Maturity	626,597	$ 0
Estimated Fair Value
Due in one year or less	0
Due from one year to five years	0
Due from five years to ten years	0
Due after ten years	154,261
Debt securities, held-to-maturity, maturity, Allocated and single maturity date, fair value	154,261
Estimated Fair Value, Held to Maturity	622,852
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	820,150
Amortized Cost	820,150
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	797,234
Estimated Fair Value	797,234
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	0
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	0
Commercial Mortgage Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	251,969
Amortized Cost	251,969
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	245,751
Estimated Fair Value	245,751
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	0
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	0
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	497,614
Amortized Cost	497,614
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	478,177
Estimated Fair Value	478,177
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	470,330
Amortized Cost, Held to Maturity	470,330
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	468,591
Estimated Fair Value, Held to Maturity	$ 468,591