CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2013	$ 4,801,999	$ 0	$ 3,441,290	$ 1,384,047	$ (23,338)
Balance (in shares) at Dec. 31, 2013		100
Increase (Decrease) in Stockholder's Equity
Net income	455,188			455,188
Other comprehensive income (loss)	(540)				(540)
Common dividends declared to parent	(452,306)			(452,306)
Contribution of capital by parent	81,498		81,498
Ending Balance at Dec. 31, 2014	4,885,839	$ 0	3,522,788	1,386,929	(23,878)
Balance (in shares) at Dec. 31, 2014		100
Increase (Decrease) in Stockholder's Equity
Net income	466,802			466,802
Other comprehensive income (loss)	42				42
Common dividends declared to parent	(330,567)			(330,567)
Contribution of capital by parent	98,036		98,036
Ending Balance at Dec. 31, 2015	$ 5,120,152	$ 0	3,620,824	1,523,164	(23,836)
Balance (in shares) at Dec. 31, 2015	100	100
Increase (Decrease) in Stockholder's Equity
Net income	$ 463,491			463,491
Other comprehensive income (loss)	836				836
Common dividends declared to parent	(327,416)			(327,416)
Contribution of capital by parent	12,392		12,392
Ending Balance at Dec. 31, 2016	$ 5,269,455	$ 0	$ 3,633,216	$ 1,659,239	$ (23,000)
Balance (in shares) at Dec. 31, 2016	100	100