Summary of Significant Accounting Policies (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
Class of Stock [Line Items]
Cash maturity period	3 months
Non-accrual status threshold	120 days
Charge off threshold	120 days
Maximum period for loan classified as non-accrual loan	120 days
Expected dividend yield		0.00%	0.00%
Stock-based compensation and related tax effects	$ 80,202	$ 79,803	$ 56,005
Monte Carlo simulation, weighted average stock price period	20 days
Stock options
Class of Stock [Line Items]
Expected dividend yield	0.00%
Stock awards, vesting period	4 years
Employee Stock
Class of Stock [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Unrecognized compensation cost expected period for recognition	6 months
Computer Software, Intangible Asset
Class of Stock [Line Items]
Intangible assets, useful life	3 years
Accumulated Deficit
Class of Stock [Line Items]
Stock-based compensation and related tax effects	$ (1,397)
Accounting Standards Update 2016-09
Class of Stock [Line Items]
Deferred tax assets				$ 56,700
Accounting Standards Update 2016-09 | Accumulated Deficit
Class of Stock [Line Items]
Stock-based compensation and related tax effects	$ 1,400
Minimum
Class of Stock [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Class of Stock [Line Items]
Property and equipment, estimated useful life	5 years