Derivative Instruments (Narrative) (Details) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended	10 Months Ended	12 Months Ended
	Sep. 30, 2010 USD ($) counterparties $ / shares shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2016 USD ($) shares	Dec. 31, 2015 USD ($) $ / shares shares
Derivative [Line Items]
Derivative collateral		$ 665.7	$ 665.7	$ 349.8
Credit risk, maximum exposure		$ 64.2		$ 36.9
Shares of common stock issued to settle warrants that have reached their expiration | shares		92,998	140,866
2015 Notes Hedges [Member]
Derivative [Line Items]
Number of counterparties | counterparties	2
Derivative, conversion spread, shares of common stock | shares	16,000,000
Derivative asset, strike price | $ / shares	$ 12.50
2015 Warrants [Member]
Derivative [Line Items]
Warrants, number of securities called by warrants | shares	16,000,000			1,600,000
Warrants, strike price | $ / shares	$ 16.00			$ 15.59
Proceeds from issuance of warrants	$ 15.6
September 2015 Notes [Member]
Derivative [Line Items]
Principal amount of convertible senior notes payable at issuance	$ 200.0
Debt instrument, interest rate, stated percentage	3.50%
SunTrust [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Interest rate swaps and caps, duration		7 years
Collateral on deposit		$ 4.5	$ 4.5
SunTrust [Member] | Three Month London Interbank Offered Rate (LIBOR) [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, cap interest rate		2.50%	2.50%
Call Options [Member]
Derivative [Line Items]
Investment maturity period		1 year