Income Taxes Deferred Tax (Liabilities) Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 33,800
Deferred tax assets:
Allowance for loan losses	84,812	$ 110,248
Deferred Tax Assets, Other Comprehensive Loss	12,404	0
Other accruals not currently deductible	7,165	20,502
Share-based compensation expense	9,418	15,498
State income taxes	9,186	12,682
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	8,336	6,799
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Deferred Rent	8,169	10,050
Loan fee income	1,189	8,266
Net operating loss	2,300	4,116
Other	3,639	2,168
Deferred tax assets	146,618	190,329
Valuation allowance	(2,624)	(4,440)
Net deferred tax assets after valuation allowance	143,994	185,889
Deferred tax liabilities:
DeferredTaxLiabilities,ChangeInAccountingMethod	(15,953)	(35,262)
Deferred Tax Liabilities, Investments	(11,145)	0
Derivative equity warrant assets	(29,127)	(36,406)
Net unrealized gains on available-for-sale securities	0	(17,970)
Deferred Tax Assets, Property, Plant and Equipment	(9,223)	(11,956)
Nonmarketable securities	(10,724)	(6,075)
Other	(3,977)	(6,380)
Deferred tax liabilities	(80,149)	(114,049)
Net deferred tax (liabilities) assets	$ 63,845	$ 71,840