Stockholders' Equity (Details) $ / shares in Units, shares in Millions		12 Months Ended
	Apr. 05, 2017 $ / shares shares	Dec. 31, 2017 USD ($) vote $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)
Class of Stock [Line Items]
Common stock par value (in Dollars per share) | $ / shares		$ 0.0005	$ 0.0005
Common stock, number of votes per share | vote		1
Repayments of Long-term Debt		$ 404,438,000	$ 835,000,000	$ 20,000,000
Payments for purchases of treasury stock		41,272,000	$ 58,961,000	$ 509,049,000
Share repurchase program, authorized amount		1,200,000,000
Stock repurchase program, remaining authorized repurchase amount		$ 1,100,000,000
CEB
Class of Stock [Line Items]
Business acquisition, shares issued as consideration | shares	7.4
Business acquisition, consideration transferred, per share value (in dollars per share) | $ / shares	$ 109.65