Commitments and Contingencies - Letters of credit (Details) (Letter of Credit, USD $)					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Restricted cash and cash equivalents	Dec. 31, 2013 Restricted cash and cash equivalents	Mar. 31, 2014 BNP Paribas	Mar. 31, 2014 Citibank	Mar. 31, 2014 J.P. Morgan
Line of Credit Facility [Line Items]
Facility	$ 300,000,000				$ 100,000,000	$ 150,000,000	$ 50,000,000
Notice period (Unused Facility Portion)					60 days	90 days	60 days
Letters of credit outstanding	131,200,000	127,300,000
Letters of credit outstanding as percentage of total facility	43.70%	42.40%
Cash and cash equivalents pledged as security			$ 131,600,000	$ 100,600,000