Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 27, 2015
Stockholders' Equity [Line Items]
Value of common stock repurchased	$ 190.4	$ 247.6	$ 116.3
Remaining shares available for purchase (in shares)	17,800,000
Common stock, shares authorized (in shares)	600,000,000.0	600,000,000.0
Cash dividends declared	$ 0	$ 0	$ 0
Additional Authorization of Shares For Repurchase [Member]
Stockholders' Equity [Line Items]
Authorized stock repurchase, shares				15,000,000
Cumulative Number Of Shares Authorized For Repurchase [Member]
Stockholders' Equity [Line Items]
Authorized stock repurchase, shares	90,000,000