Loans and Allowance for Credit Losses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, 90 Days or More Past Due, Still Accruing	$ 595,000		$ 595,000				$ 531,000
Direct Financing Lease, Lease Income	5,300	$ 5,700	16,000	$ 16,500
Financing Receivable, before Allowance for Credit Loss and Fee	23,779,315		23,779,315				23,154,988
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)			(125,000)
Purchases of loans			2,082,695	1,636,932
Recorded investment of loans pledged as security for FHLB advances and Federal Reserve discount window borrowings	10,900,000		10,900,000				10,200,000
Financing Receivable, Allowance for Credit Loss	(274,128)	(108,462)	(274,128)	(108,462)	$ (266,123)		(108,671)	$ (112,141)	$ (109,931)
Interest Receivable	105,000		105,000				83,000
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	2,700	2,000	7,600	6,000
Accounts, Notes, Loans and Financing Receivable [Line Items]
Capital Leases, Net Investment in Direct Financing Leases, Initial Direct Costs	687,000		687,000				733,000
Small business finance [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, Nonaccrual	43,600		43,600				45,700
Financing Receivable, 90 Days or More Past Due, Still Accruing	33,600		33,600				36,300
Financial Asset Acquired with Credit Deterioration [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	126,000		126,000
Financing Receivable, Allowance for Credit Loss	(1,500)		(1,500)			$ (1,700)
1-4 single family residential
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	4,845,156		4,845,156				4,953,936
Purchases of loans	997,000	743,000	2,100,000	1,600,000
Residential Portfolio Segment [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	5,940,900		5,940,900				5,661,119
Mortgage Loans in Process of Foreclosure, Amount	270,000		270,000				257,000
Other Real Estate, Foreclosed Assets, and Repossessed Assets							6,000
Financing Receivable, Allowance for Credit Loss	(16,030)	(11,399)	(16,030)	(11,399)	(10,695)		(11,154)	(11,236)	(10,788)
US Government Agency Insured Loans [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	1,089,055		1,089,055				698,644
Purchases of loans	418,000	288,000	947,000	572,000
Mortgage Loans in Process of Foreclosure, Amount	264,000		264,000				248,000
Commercial Portfolio Segment [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	17,838,415		17,838,415				17,493,869
Financing Receivable, Allowance for Credit Loss	(258,098)	$ (97,063)	(258,098)	$ (97,063)	$ (255,428)		$ (97,517)	$ (100,905)	$ (99,143)
Commercial Portfolio Segment [Member] | Minimum
Financing Receivable Investment Past Due [Line Items]
Financing receivable credit quality indicator, the value at which relationships are reevaluated at least annually	1,000		1,000
Commercial Portfolio Segment [Member] | Maximum [Member]
Financing Receivable Investment Past Due [Line Items]
Financing receivable credit quality indicator, the value at which relationships are reevaluated at least annually	$ 3,000		$ 3,000