Investment Securities Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jul. 01, 2019
Schedule of Investments [Line Items]
Amortized cost	$ 2,691,879			$ 2,691,879
Available-for-sale Securities, Fair Value Disclosure	2,793,480			2,793,480		$ 2,497,537
HTM, at amortized cost	304,241			304,241		369,841
HTM securities	324,940			324,940
Prepayment penalty on FHLB advances	0		$ 4,326	2,878	$ 4,326
Debt Securities [Member]
Schedule of Investments [Line Items]
HTM, at amortized cost	79,000		409,200	79,000	409,200
HTM securities	82,000		413,700	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 [Member]
Schedule of Investments [Line Items]
Amortized cost	318,343			318,343
Available-for-sale Securities, Fair Value Disclosure	321,695			321,695
HTM, at amortized cost	304,241			304,241		369,841
HTM securities	324,940			324,940		383,705
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized cost	880,495			880,495
Available-for-sale Securities, Fair Value Disclosure	926,554			926,554
US States and Political Subdivisions Debt Securities [Member] | Accounting Standards Update 2019-04
Schedule of Investments [Line Items]
HTM, at amortized cost							$ 158,900
HTM securities							$ 168,500
Collateral Pledged [Member]
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 568,400			$ 568,400		$ 462,600