Investments Income from Equity Investments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Income from equity investments, net, before amortization of basis difference									$ 190,864	$ 202,758	$ 201,207
Amortization of basis difference									(53,502)	(54,602)	(54,248)
Income on equity investments, net	$ 38,506	$ 21,058	$ 40,761	$ 37,037	$ 33,861	$ 31,737	$ 44,306	$ 38,252	$ 137,362	$ 148,156	$ 146,959