Investments - Aggregated Statement of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Aggregated statement of operations [Abstract]
Operating revenues	$ 1,973,515	$ 1,333,239	$ 1,263,231
Operating income	753,249	708,958	666,227
Net income	$ 568,358	$ 522,435	$ 483,143