Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 7,111	$ 4,022
Total gains (losses) included in earnings:
Issuances	24,763	31,188
Settlements and closed loans	(28,018)	(25,963)
Expirations	312	(2,136)
Ending balance	4,168	7,111
Other Liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1,065)	(651)
Total gains (losses) included in earnings:
Issuances	(3,924)	(10,698)
Settlements and closed loans	4,298	10,284
Expirations	0	0
Ending balance	$ (691)	$ (1,065)