Equity - Treasury Stock (Details) - USD ($) shares in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired	$ 11,953,000	$ 173,913,000
Cash payments related to stock repurchases	$ 19,933,000	$ 173,913,000
2017 and 2018 Repurchase Programs
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired (in shares)	0.4	5.0	13.9
Treasury stock acquired	$ 12,000,000.0	$ 173,900,000	$ 451,300,000
2017 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Aggregate authorized amount of common stock to be repurchased (up to)					$ 300,000,000.0
2018 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Aggregate authorized amount of common stock to be repurchased (up to)				$ 500,000,000.0
Amount remaining under stock repurchase programs	286,800,000
Treasury Stock Associated with Deferred Compensation Plans
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired	$ 3,800,000	$ 3,300,000
Common Stock Withheld for Settlement of Employee Tax Liabilities
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired (in shares)	0.4	0.4
Treasury stock acquired	$ 15,300,000	$ 13,400,000