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	6 Months Ended	12 Months Ended
	Jun. 30, 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 market servicing rate assumptions, servicing assets	0.66%	0.66%
Weighted-average market servicing rate assumptions, servicing liabilities	0.66%	0.66%
Assets, servicing rate increase by .1%	$ (9,431)	$ (7,749)
Liabilities, servicing rate increase by .1%	101	233
Assets, servicing rate decrease by .1%	9,437	7,760
Liabilities, servicing rate decrease by .1%	$ (95)	$ (222)