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, 2013	$ 604,282	$ 93,300	$ 248,844	$ 262,499	$ (361)
Balance (in shares) at Dec. 31, 2013		933,000
Comprehensive income:
Net income	70,642			70,642
Other comprehensive income	76				76
Common dividends declared to parent	(50,743)			(50,743)
Contribution of capital by parent	73,432		73,432
Ending 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	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