Guarantees and Long Term Standby Purchase Commitments - Guarantee and Commitment Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Term of Guarantees Minimum	1 year
Term of Guarantees Maximum	30 years
Additions to guarantee and commitment obligations	$ 6,202	$ 7,683	$ 6,725
Beginning Balance Guaranty Liabilities	38,400	37,282	38,609
Ending Balance Guaranty Liabilities	38,683	38,400	37,282
Amortization of the Guarantee and Commitment Obligation	(5,919)	(6,565)	$ (8,052)
Guarantee obligations issued after January 1, 2003 [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	4,000,000	4,000,000
Guarantee obligations issued prior to January 1, 2003 [Member]
Loss Contingencies [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 23,800	$ 28,000