Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Money market funds	$ 52,300,000
Threshold period of delinquency	90 days
Market spread	one-month LIBOR	Index Rate
Transfers of financial assets or liabilities with in fair value hierarchy	$ 0	$ 0
Estimated fair value of loans held for investment	$ 4,700,000,000	$ 4,300,000,000
Weighted average gross spread percentage	3.90%	3.90%
Weighted average maturity period	3 years 9 months 18 days	3 years 10 months 24 days

Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

No definition available.

Name:	us-gaap_MoneyMarketFundsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant