Marketable Securities - Securitization Trusts (Details) (Securitization Trust, Primary Beneficiary [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Securitization Trust, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Size/Principal outstanding	$ 3,952,037	[1]			$ 4,062,068	[1]
Assets on Balance Sheet	3,201	[2]			3,381	[2]
Liabilities on Balance Sheet	0				0
Maximum exposure to loss	3,201	[3]			3,381	[3]
Year to date loss on sale	0				0
Year to date cash flows	$ 1,597	[4]	$ 1,821	[4]

[1] rincipal Outstanding reflects the estimated principal of the underlying assets held by the securitization trust.

[2] Assets on balance sheet are securities issued by the entity and are recorded in the current marketable securities line item on the condensed consolidated balance sheets.

[3] The maximum exposure to loss includes the assets held by the Company. The maximum exposure to loss assumes a total loss on the referenced assets held by the securitization trust.

[4] Year to date cash flows are for the three months ended March 31, 2015 and 2014, respectively.