Allowance for Credit Losses (Components of Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	$ 373,109	$ 253,412	$ 158,461	$ 161,901	$ 159,622	$ 154,164
Allowance for held-to-maturity securities losses	65		0	0
Allowance for credit losses	373,174		158,461	161,901
Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	313,510		156,828	160,421
Unfunded lending-related commitments
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	$ 59,599		$ 1,633	$ 1,480