Allowance for Credit Losses (Components of Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	$ 388,908	$ 373,109	$ 158,461	$ 163,273	$ 161,901	$ 154,164
Allowance for held-to-maturity securities losses	63		0	0
Allowance for credit losses	388,971		158,461	163,273
Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	325,959		156,828	161,763
Unfunded lending-related commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	$ 62,949		$ 1,633	$ 1,510