Fair Value of Financial Instruments - Changes in Unrealized Gains (Losses) Included in Earnings for Level 3 (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Change in unrealized gains (losses) – assets									$ (61,957)	$ 390,371	$ (423,949)
Change in unrealized (losses) gains – liabilities									74,325	(333,440)	455,444
Net change in unrealized gains included in earnings for assets and liabilities	$ (4,508)	$ (2,170)	$ 5,418	$ 13,628	$ 13,871	$ 18,115	$ 20,019	$ 4,926	$ 12,368	$ 56,931	$ 31,495