Investment Securities Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Aug. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 01, 2019
HTM, at amortized cost		$ 278,281	$ 369,841
HTM securities		296,857
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost	$ 665,500
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value	641,700
Amortized cost basis		2,947,500
Estimated Fair Value		3,062,143	2,497,537
Prepayment penalty on FHLB advances		2,878	4,326	$ 0
Balance of cumulative credit losses on debt securities		990	990	$ 11,500
Collateral Pledged
Available-for-sale securities pledged as collateral		520,500	462,600
Debt securities
HTM, at amortized cost		79,000	409,200
HTM securities		82,000	413,700
Debt Securities, Held-to-maturity, Sold, Realized Gain (Loss)		3,000	4,500
Prepayment penalty on FHLB advances		2,900	4,300
Residential mortgage-backed securities
HTM, at amortized cost		278,281	369,841
HTM securities		296,857	383,705
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost	505,500
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value	485,300
Amortized cost basis		373,779
Estimated Fair Value		377,998
Auction rate securities
Amortized cost basis		101,510
Estimated Fair Value		98,206
State and municipal securities
HTM, at amortized cost			369,841
HTM securities			$ 383,705
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Amortized Cost	160,000
Debt Securities, Available-For-Sale, Transfer To Held-To-Maturity, Fair Value	$ 156,400
Amortized cost basis		891,327
Estimated Fair Value		$ 952,613
Accounting Standards Update 2019-04 | State and municipal securities
HTM, at amortized cost					$ 158,900
HTM securities					$ 168,500