Stockholders' Equity (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Accelerated Share Repurchases [Line Items]
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares outstanding (in shares)	79,553,825	79,465,393
Shares available for grant (in shares)	4,053,539
Purchases and retirement of common stock	$ 0	$ 212,500,000	$ 287,500,000
Remaining authorized repurchase amount	331,400,000
Common stock, quarterly dividends declared (in dollars per share)					$ 0.14	$ 0.13	$ 0.12
2012, 2013, 2014 and 2016 Share Repurchase Program [Member]
Accelerated Share Repurchases [Line Items]
Authorized amount	1,350,000,000
Accelerated Share Repurchase
Accelerated Share Repurchases [Line Items]
Purchases and retirement of common stock		$ 212,500,000	$ 287,500,000
Purchases and retirement of common stock (in shares)		4,413,250	5,541,930
December 2016 ASR Program
Accelerated Share Repurchases [Line Items]
Authorized amount	$ 300,000,000.0
Forecast
Accelerated Share Repurchases [Line Items]
Common stock, quarterly dividends declared (in dollars per share)				$ 0.15