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, 2017	$ 876,600	$ 93,300	$ 449,400	$ 334,000	$ (100)
Balance (in shares) at Dec. 31, 2017		933,000
Increase (Decrease) in Stockholder's Equity
Net income	31,400			31,400
Other comprehensive income	0				0
Common dividends declared to parent	(16,000)			(16,000)
Contribution of capital by parent	0		0
Ending Balance at Mar. 31, 2018	892,000	$ 93,300	449,400	349,400	(100)
Balance (in shares) at Mar. 31, 2018		933,000
Beginning Balance at Dec. 31, 2018	$ 942,400	$ 93,300	510,100	339,000	0
Balance (in shares) at Dec. 31, 2018	933,000	933,000
Increase (Decrease) in Stockholder's Equity
Net income	$ 24,000			24,000
Other comprehensive income	0				0
Common dividends declared to parent	(24,400)			(24,400)
Contribution of capital by parent	0		0
Ending Balance at Mar. 31, 2019	$ 942,000	$ 93,300	$ 510,100	$ 338,600	$ 0
Balance (in shares) at Mar. 31, 2019	933,000	933,000