Equity - Share Repurchase Program and Share Repurchase Activity (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 05, 2017	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 1,200,000,000.0		$ 1,200,000,000.0
Stock repurchase program, remaining authorized repurchase amount			$ 1,100,000,000		$ 1,100,000,000
Number of shares repurchased (in shares)			26,725	9,882	348,236	542,792
Cash paid for repurchased shares (in thousands)		$ 7,200,000	$ 3,275,000	$ 922,000	$ 37,188,000	$ 52,889,000
Treasury stock, average price paid per share (in dollars per share)			$ 122.53	$ 93.33	$ 106.79	$ 84.17
CEB
Class of Stock [Line Items]
Business acquisition, equity interest issued, number of shares (in shares)	7,400,000
Business combination, equity interest issued, per share fair value (in dollars per share)	$ 109.65