STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning Balance at Dec. 31, 2013	$ 1,363,691	$ 0	$ 1,005,463	$ 359,389	$ (1,161)
Beginning Balance (in shares) at Dec. 31, 2013		100
Comprehensive income:
Net income	129,852			129,852
Other comprehensive income	172				172
Common dividends declared to parent	(93,243)			(93,243)
Contribution of capital by parent	160,000		160,000
Ending Balance at Dec. 31, 2014	1,560,472	$ 0	1,165,463	395,998	(989)
Ending Balance (in shares) at Dec. 31, 2014		100
Comprehensive income:
Net income	127,263			127,263
Other comprehensive income	(292)				(292)
Common dividends declared to parent	(85,254)			(85,254)
Contribution of capital by parent	205,760		205,760
Ending Balance at Dec. 31, 2015	$ 1,807,949	$ 0	1,371,223	438,007	(1,281)
Ending Balance (in shares) at Dec. 31, 2015	100	100
Comprehensive income:
Net income	$ 152,157			152,157
Other comprehensive income	(9)				(9)
Common dividends declared to parent	(103,401)			(103,401)
Contribution of capital by parent	75,000		75,000
Ending Balance at Dec. 31, 2016	$ 1,931,696	$ 0	$ 1,446,223	$ 486,763	$ (1,290)
Ending Balance (in shares) at Dec. 31, 2016	100	100