Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Correction of Prior Period Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other income	$ 23,067	$ 18,949	$ 16,002
Other operating expense	$ 41,200	34,416	29,063
Immaterial Error, Cash Servicing Fees
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other income		18,949	16,002
Other operating expense		34,416	29,063
Immaterial Error, Cash Servicing Fees | Scenario, Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other income		16,243	14,001
Other operating expense		31,710	27,062
Immaterial Error, Cash Servicing Fees | Scenario, Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Other income		2,706	2,001
Other operating expense		$ 2,706	$ 2,001