Business and Significant Accounting Policies - Schedule of Contract with Customer, Asset and Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Fees receivable, gross	$ 1,262,818	$ 1,162,871
Contract assets	26,119	26,672
Contract liabilities:
Deferred revenues (current liability)	1,745,244	1,630,198
Non-current deferred revenues	21,194	16,205
Total contract liabilities	1,766,438	1,646,403
Revenue recognized previously attributable to deferred revenues	1,287,800
Revenue reserve balance	$ 7,400
Amortization of deferred sales commissions, amortization period	1 year
Amortization of deferred sales commissions	$ 304,800	$ 230,500	$ 180,200