Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 6,738
Due from one year to five years	69,672
Due from five years to ten years	108,091
Due after ten years	348,332
Amortized cost, before securities without debt maturities	532,833
Amortized cost basis	2,115,265
Due in one year or less	0
Due from five years to ten years	0
Due from five years to ten years	0
Due after ten years	156,134
Amortized cost, before securities without debt maturities	156,134
Amortized cost basis	606,679	$ 0
Estimated Fair Value
Due in one year or less	6,740
Due from one year to five years	70,182
Due from five years to ten years	106,709
Due after ten years	339,623
Available for sale securities, debt maturities, before securities without single maturities	523,254
Estimated Fair Value	2,080,294	$ 2,547,956
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	157,207
Held to maturity securities, before securities without single maturities	157,207
Debt Securities, Held-to-maturity	611,419
Residential mortgage-backed securities
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	476,973
Amortized cost basis	476,973
Held to maturity securities, amortized cost without single maturity date	450,545
Amortized cost basis	450,545
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	463,344
Estimated Fair Value	463,344
Held to maturity securities, without single maturities	454,212
Debt Securities, Held-to-maturity	454,212
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	264,165
Amortized cost basis	841,294
Held to maturity securities, amortized cost without single maturity date	0
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	261,616
Estimated Fair Value	832,080
Held to maturity securities, without single maturities	0
Residential mortgage-backed securities
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	841,294
Amortized cost basis	264,165
Held to maturity securities, amortized cost without single maturity date	0
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	832,080
Estimated Fair Value	261,616
Held to maturity securities, without single maturities	$ 0