Summary of Significant Accounting Policies Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Other-than-temporary impairment on available-for-sale securities	$ 0	$ 1,600,000	$ 1,300,000
Fully depreciated assets taken out of service	4,800,000	1,400,000
Property and equipment asset impairments	$ 0	0	0
Number of reporting units	1
Number of operating segments	1
Goodwill impairments	$ 0	0	0
Indefinite-lived intangible asset impairments	0	0	0
Finite-lived intangible asset impairments	$ 0	0	0
Deferred sales commission asset estimated period of benefit	8 years
Amortization of deferred sales commissions	$ 8,025,000	11,980,000	15,054,000
Deferred sales commission asset impairments	0	0	0
Total waived fees	310,800,000	413,700,000	662,700,000
Change in Voluntary Yield-related Fee Waivers	(83,400,000)
Advertising Expense	$ 1,500,000	$ 2,700,000	$ 2,600,000
Minimum [Member]
Property and equipment, useful Life	1 year
Maximum [Member]
Property and equipment, useful Life	15 years
Software and Software Development Costs [Member]
Property and equipment, useful Life	4 years