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Summary of Significant Accounting Policies Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Fully depreciated assets taken out of service $ 1,400 $ 10,400  
Deferred Sales Commission Asset B-Shares Estimated Period Of Benefit 8 years    
Amortization of deferred sales commissions $ 11,980 15,054 $ 12,699
Total Waived Fees 413,700 662,700 764,300
Change in Voluntary Yield-related Fee Waivers (245,800)    
Change due to competitive waivers (5,200)    
Advertising Expense $ 2,700 $ 2,600 $ 2,200
Number of operating segments 1    
Minimum [Member]      
Property and equipment, useful Life 1 year    
Maximum [Member]      
Property and equipment, useful Life 12 years    
Software and Software Development Costs [Member]      
Property and equipment, useful Life 4 years