Summary of Significant Accounting Policies - Effects of New Accounting Standards on Previously Reported Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by Operating Activities	$ 898,160	$ 397,441	$ 749,598
Net Cash Provided by Financing Activities	$ (210,502)	453,805	223,296
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by Operating Activities		397,441	749,598
Net Cash Provided by Financing Activities		453,805	223,296
Accounting Standards Update 2016-09 | As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net Cash Provided by Operating Activities		392,377	740,737
Net Cash Provided by Financing Activities		$ 458,869	$ 232,157