Loans and Allowance for Credit Losses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, 90 Days or More Past Due, Still Accruing	$ 673,000		$ 673,000				$ 531,000
Direct Financing Lease, Lease Income	5,300	$ 5,500	10,700	$ 10,800
Financing Receivable, before Allowance for Credit Loss and Fee	23,834,889		23,834,889				23,154,988
Financing Receivable, Purchased with Credit Deterioration, Discount (Premium)			(135,000)
Purchases of loans			1,085,437	894,235
Recorded investment of loans pledged as security for FHLB advances and Federal Reserve discount window borrowings	11,000,000		11,000,000				10,200,000
Unpaid principal balance of ACI loans							408,000
Financing Receivable, Allowance for Credit Loss	(266,123)	(112,141)	(266,123)	(112,141)	$ (250,579)		(108,671)	$ (114,703)	$ (109,931)
Interest Receivable	108,000		108,000				83,000
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	3,000	2,500	5,700	4,300
Small business finance [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, Nonaccrual	45,700		45,700
Financing Receivable, 90 Days or More Past Due, Still Accruing	37,300		37,300				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	134,000		134,000
Financing Receivable, Allowance for Credit Loss	(1,100)		(1,100)			$ (1,700)
1-4 single family residential
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	4,743,866		4,743,866				4,953,936
Purchases of loans	582,000	589,000	1,100,000	894,000
Residential Portfolio Segment [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	5,577,807		5,577,807				5,661,119
Mortgage Loans in Process of Foreclosure, Amount	323,000		323,000				257,000
Other Real Estate, Foreclosed Assets, and Repossessed Assets	0		0				6,000
Financing Receivable, Allowance for Credit Loss	(10,695)	(11,236)	(10,695)	(11,236)	(12,576)		(11,154)	(10,952)	(10,788)
US Government Agency Insured Loans [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	826,238		826,238				698,644
Purchases of loans	243,000	151,000	529,000	284,000
Mortgage Loans in Process of Foreclosure, Amount	315,000		315,000				248,000
Commercial Portfolio Segment [Member]
Financing Receivable Investment Past Due [Line Items]
Financing Receivable, before Allowance for Credit Loss and Fee	18,257,082		18,257,082				17,493,869
Financing Receivable, Allowance for Credit Loss	(255,428)	$ (100,905)	(255,428)	$ (100,905)	$ (238,003)		$ (97,517)	$ (103,751)	$ (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