Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 5,828
Due from one year to five years	38,664
Due from five years to ten years	175,654
Due after ten years	390,033
Amortized cost, before securities without debt maturities	610,179
Amortized Cost	2,263,825
Estimated Fair Value
Due in one year or less	5,828
Due from one year to five years	39,900
Due from five years to ten years	178,922
Due after ten years	392,471
Available for sale securities, debt maturities, before securities without single maturities	617,121
Estimated Fair Value	2,285,794	$ 2,080,294
Amortized Cost
Due in one year or less	0
Due from one year to five years	0
Due from five years to ten years	2,154
Due after ten years	153,707
Debt securities, held-to-maturity, maturity, allocated and single maturity date, amortized cost, total	155,861
Amortized Cost, Held to Maturity	567,564	606,679
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	2,277
Due after ten years	162,284
Debt securities, held-to-maturity, maturity, Allocated and single maturity date, fair value	164,561
Estimated Fair Value, Held to Maturity	589,728	611,419
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	889,053
Amortized Cost	889,053
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	899,117
Estimated Fair Value	899,117
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	433,027
Amortized Cost	433,027
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	440,281
Estimated Fair Value	440,281
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	331,566
Amortized Cost	331,566
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	329,275
Estimated Fair Value	329,275
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	411,703
Amortized Cost, Held to Maturity	411,703	450,545
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	425,167
Estimated Fair Value, Held to Maturity	$ 425,167	$ 454,212