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, 2012	$ 517,106	$ 93,300	$ 189,867	$ 234,376	$ (437)
Balance (in shares) at Dec. 31, 2012		933,000
Comprehensive income:
Net income	59,468			59,468
Other comprehensive income	76				76
Common dividends declared to parent	(31,345)			(31,345)
Contribution of capital by parent	58,977		58,977
Ending 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	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