Equity - Share Repurchase Plan, Preferred Stock (Details) $ / shares in Units, $ in Millions	12 Months Ended			276 Months Ended
	Dec. 31, 2016 item $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2016 $ / shares shares
Share Repurchase Plan
Stock Repurchase Program, Increase In Number Of Shares Authorized To Be Repurchased	7,500,000
Repurchase of common shares (in shares)	0	6,194,641	759,800	22,100,000
Average costs per share | $ / shares		$ 57.39	$ 65.80	$ 30.93
Value of shares repurchased | $		$ 355.5	$ 50.0
Remaining number of common stock authorized for repurchase under stock repurchase program (in shares)	8,428,592			8,428,592
Preferred Stock
Preferred stock, authorized shares	5,000,000	5,000,000		5,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001		$ 0.001
Preferred stock, issued shares	0	0		0
Preferred stock, outstanding shares	0	0		0
Minimum number of series in which preferred shares may be issued | item	1