Convertible Senior Notes (Interest Expense) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Disclosure [Abstract]
Contractual interest expense	$ 3,781	$ 2,101	$ 0
Amortization of debt issuance costs	1,179	598	0
Amortization of debt discount	9,978	5,306	0
Total interest expense	$ 14,938	$ 8,005	$ 0
Effective interest rate of the liability component	6.02%	6.02%	0.00%