Equity - Share Repurchase Program and Share Repurchase Activity (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Class of Stock [Line Items]
Stock repurchase program, authorized amount	$ 1,200,000,000.0
Stock repurchase program, remaining authorized repurchase amount	$ 1,100,000,000
Number of shares repurchased (in shares)	239,268	218,752
Cash paid for repurchased shares (in thousands)	$ 28,394,000	$ 21,978,000
Treasury stock, average price paid per share (in dollars per share)	$ 118.73	$ 100.47