Allowance for Credit Losses (Held-to-Maturity Debt Securities by Credit Quality Indication) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 728,530	$ 1,134,400	$ 1,191,634
Less: Allowance for credit losses	(65)	0	0
Held-to-maturity securities, net of allowance for credit losses	728,465	1,134,400	1,191,634
U.S. Government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	124,575
2019	284,973
2018	101,450
2017	0
2016	0
Prior	3,406
Amortized Cost	514,404	902,974	952,526
U.S. Government agencies | 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	124,575
2019	284,973
2018	101,450
2017	0
2016	0
Prior	3,406
Amortized Cost	514,404
U.S. Government agencies | 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
U.S. Government agencies | 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
Municipal
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	162
2018	7,611
2017	44,073
2016	10,200
Prior	152,080
Amortized Cost	214,126	$ 231,426	$ 239,108
Municipal | 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	162
2018	7,611
2017	44,073
2016	10,200
Prior	152,080
Amortized Cost	214,126
Municipal | 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
Municipal | 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	$ 0