Guarantees and Long Term Standby Purchase Commitments - Guarantee and Commitment Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Term of Guarantees Minimum	1 year
Term of Guarantees Maximum	30 years
Beginning Balance Guaranty Liabilities	$ 37,282	$ 38,609	$ 37,925
Additions to the guarantee and commitment obligation(1)	7,683	6,725	8,207
Amortization of the guarantee and commitment obligation	(6,565)	(8,052)	(7,523)
Ending Balance Guaranty Liabilities	38,400	37,282	$ 38,609
Guarantee obligations issued after January 1, 2003 [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	4,000,000	4,900,000
Guarantee obligations issued prior to January 1, 2003 [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 28,000	$ 40,100