Stockholders' Equity - Registered Underwritten Public Offerings (Details) - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Sep. 21, 2016	Jun. 29, 2015	Nov. 17, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 31, 2014
Schedule of Stockholders' Equity [Line Items]
Issuance of stock (in shares)	48,787,878
Price of shares issued (in dollars per share)	$ 8.25
Issuance of stock	$ 402.5			$ 402.5
Issuance costs	$ 11.9			11.9	$ 15.0
Proceeds from issuance of common stock, net		$ 483.0	$ 403.0	$ 391.5	$ 469.5	$ 1,512.6
Share price (in dollars per share)							$ 0.01
Common Stock
Schedule of Stockholders' Equity [Line Items]
Issuance of stock (in shares)		18,226,414	16,445,000	48,787,878		9,242
Price of shares issued (in dollars per share)		$ 26.50		$ 8.25	$ 12.56
Issuance of stock				$ 0.5
Offering expenses		$ 15.0	$ 15.1
Share price (in dollars per share)			$ 24.50			$ 10.00