Securities Securities - Available for sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	$ 3,487,296	$ 3,491,633
Gross Unrealized Gain	117,222	66,147
Gross Unrealized Loss	17,837	17,785
Securities available-for-sale, at fair value	3,586,681	3,539,995
Available-for-sale, Amortized Cost
Due in one year of less	83,050
Due in one year to five years	12,406
Due in five to ten years	188,991
Due after ten years	1,401,212
Mortgage-backed securities	1,623,759
Asset-backed securities	177,878
Amortized Cost	3,487,296
Available-for-sale, Fair Value
Due in one year or less	83,066
Due in one year to five years	12,802
Due in five years to ten years	198,687
Due after ten years	1,424,999
Mortgage-backed securities	1,689,191
Asset-backed securities	177,936
Securities available-for-sale, at fair value	3,586,681	3,539,995
State and Municipal Securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	1,411,288	1,669,606
Gross Unrealized Gain	44,559	52,096
Gross Unrealized Loss	12,484	7,249
Securities available-for-sale, at fair value	1,443,363	1,714,453
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	1,443,363	1,714,453
U.S. Government Agency Securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	74,916	80,096
Gross Unrealized Gain	1,547	306
Gross Unrealized Loss	60	710
Securities available-for-sale, at fair value	76,403	79,692
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	76,403	79,692
Mortgage-backed Agency Securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	1,623,759	1,458,894
Gross Unrealized Gain	67,759	12,789
Gross Unrealized Loss	2,327	7,776
Securities available-for-sale, at fair value	1,689,191	1,463,907
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	1,689,191	1,463,907
US Treasury Securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	82,199	72,862
Gross Unrealized Gain	10	19
Gross Unrealized Loss	0	14
Securities available-for-sale, at fair value	82,209	72,867
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	82,209	72,867
Asset-backed Securities
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	177,878	153,963
Gross Unrealized Gain	715	302
Gross Unrealized Loss	657	1,293
Securities available-for-sale, at fair value	177,936	152,972
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	177,936	152,972
Corporate Notes
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	117,256	56,212
Gross Unrealized Gain	2,632	635
Gross Unrealized Loss	2,309	743
Securities available-for-sale, at fair value	117,579	56,104
Available-for-sale, Fair Value
Securities available-for-sale, at fair value	$ 117,579	$ 56,104