Investment Securities Schedule of Amortized Cost and Fair Values of Debt Securities by Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 13,107
Due from one year to five years	37,176
Due from five years to ten years	343,642
Due after ten years	984,188
Amortized cost, before securities without debt maturities	1,378,113
Amortized Cost	2,693,919
Estimated Fair Value
Due in one year or less	13,171
Due from one year to five years	38,417
Due from five years to ten years	355,683
Due after ten years	1,023,064
Available for sale securities, debt maturities, before securities without single maturities	1,430,335
Estimated Fair Value	2,758,597	$ 3,062,143
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	0
Debt securities, held-to-maturity, maturity, allocated and single maturity date, amortized cost, total	0
Amortized Cost, Held to Maturity	853,413	278,281
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	0
Debt securities, held-to-maturity, maturity, Allocated and single maturity date, fair value	0
Estimated Fair Value, Held to Maturity	843,743
Collateralized Mortgage Obligations [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	390,253
Amortized Cost	390,253
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	399,881
Estimated Fair Value	399,881
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	668,144
Amortized Cost	668,144
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	673,017
Estimated Fair Value	673,017
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	385,952
Amortized Cost, Held to Maturity	385,952
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	368,004
Estimated Fair Value, Held to Maturity	368,004
Mortgage-Backed Securities [Member]
Amortized Cost
Available-for-sale securities, amortized cost without single maturity date	257,409
Amortized Cost	257,409
Estimated Fair Value
Available-for-sale securities, debt maturities, without single maturity date, fair value	255,364
Estimated Fair Value	255,364
Amortized Cost
Debt securities, held-to-maturity, maturity, without single maturity date, amortized cost	467,461
Amortized Cost, Held to Maturity	467,461	278,281
Estimated Fair Value
Debt securities, held-to-maturity, maturity, without single maturity date, fair value	475,739
Estimated Fair Value, Held to Maturity	$ 475,739	$ 296,857