Loans Receivable and Allowance for Credit Losses (Allowance for Credit Losses Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable Allowance for Credit Losses
Allowance for loan losses	$ 557,003,000	$ 317,894,000		$ 358,287,000
Allowance for loan losses, percentage of loans held-for-investment	1.55%	0.97%		1.03%
Increase in provision for credit losses		$ 73,900,000
Gross charge-offs	$ 0	73,000
PCD Loans
Financing Receivable Allowance for Credit Losses
Allowance for loan losses		$ 14,000		$ 0	$ 22,000
Commercial and consumer lending
Financing Receivable Allowance for Credit Losses
Allowance for loan losses	557,003,000		$ 483,445,000	$ 358,287,000
Gross charge-offs	12,957,000
Net charge-offs	$ (898,000)
Net charge-offs, percentage of loans held-for-investment	0.01%
Commercial and industrial (“C&I”)
Financing Receivable Allowance for Credit Losses
Allowance for loan losses	$ 416,000
Gross charge-offs	$ 12,000,000.0
ASU 2016-13
Financing Receivable Allowance for Credit Losses
Allowance for loan losses			125,200,000
ASU 2016-13 | PCD Loans
Financing Receivable Allowance for Credit Losses
Allowance for loan losses			$ 1,200,000