Fair Value Measurements - Aggregate Fair Value, Contractual Balance, and Gain or Loss for Loans Held-For-Sale (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Aggregate fair value	$ 31,636	$ 52,133
Contractual balance	30,800	50,597
Unrealized gain (loss)	$ 836	$ 1,536