Fair Value of Assets and Liabilities - Additional Information about Servicing Assets and Liabilities Measured Using Different Market Servicing Rates and Different Prepayment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in rate	0.10%	0.10%
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted-average total market servicing rate assumptions	0.66%	0.66%
Assets, servicing rate increase by .1%	$ (8,340)	$ (7,749)
Liabilities, servicing rate increase by .1%	154	233
Assets, servicing rate decrease by .1%	8,343	7,760
Liabilities, servicing rate decrease by .1%	$ (151)	$ (222)