CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2014	$ 697,689	$ 93,300	$ 322,276	$ 282,398	$ (285)
Balance (in shares) at Dec. 31, 2014		933,000
Comprehensive income:
Net income	74,636			74,636
Other comprehensive income	76				76
Common dividends declared to parent	(54,293)			(54,293)
Contribution of capital by parent	72,277		72,277
Ending Balance at Dec. 31, 2015	790,385	$ 93,300	394,553	302,741	(209)
Balance (in shares) at Dec. 31, 2015		933,000
Comprehensive income:
Net income	69,135			69,135
Other comprehensive income	76				76
Common dividends declared to parent	(48,508)			(48,508)
Contribution of capital by parent	762		762
Ending Balance at Dec. 31, 2016	$ 811,850	$ 93,300	395,315	323,368	(133)
Balance (in shares) at Dec. 31, 2016	933,000	933,000
Comprehensive income:
Net income	$ 79,416			79,416
Other comprehensive income	76				76
Common dividends declared to parent	(68,788)			(68,788)
Contribution of capital by parent	54,035		54,035
Adoption of ASU No. 2018-02	0			12	(12)
Ending Balance at Dec. 31, 2017	$ 876,589	$ 93,300	$ 449,350	$ 334,008	$ (69)
Balance (in shares) at Dec. 31, 2017	933,000	933,000