Deferred Sales Commissions, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gross to net deferred sales commissions [Abstract]
Carrying amount of deferred sales commissions	$ 911,852	$ 903,252
Less: Accumulated amortization	(597,566)	(565,681)
Cumulative CDSC received	(284,160)	(273,681)
Deferred sales commissions, net	30,126	63,890
Amortization expense	$ 31,886	$ 41,066	$ 49,145