Deferred Sales Commissions, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gross to net deferred sales commissions [Abstract]
Deferred Sales Commissions Gross	$ 903,252	$ 970,671
Less: Accumulated amortization	(565,681)	(606,963)
Cumulative CDSC received	(273,681)	(264,638)
Deferred sales commissions, net	63,890	99,070
Amortization expense	$ 41,066	$ 49,145	$ 41,508