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	$ 5,828,300	$ 0	$ 4,032,800	$ 1,822,200	$ (26,700)
Balance (in shares) at Dec. 31, 2017		100
Increase (Decrease) in Stockholder's Equity
Net income	133,700			133,700
Other comprehensive income	300				300
Common dividends declared to parent	(95,300)			(95,300)
Contribution of capital by parent	0		0
Ending Balance at Mar. 31, 2018	5,867,000	$ 0	4,032,800	1,860,600	(26,400)
Balance (in shares) at Mar. 31, 2018		100
Beginning Balance at Dec. 31, 2018	$ 6,298,200	$ 0	4,340,500	1,983,200	(25,500)
Balance (in shares) at Dec. 31, 2018	100	100
Increase (Decrease) in Stockholder's Equity
Net income	$ 138,800			138,800
Other comprehensive income	300				300
Common dividends declared to parent	(98,800)			(98,800)
Contribution of capital by parent	50,000		50,000
Ending Balance at Mar. 31, 2019	$ 6,388,500	$ 0	$ 4,390,500	$ 2,023,200	$ (25,200)
Balance (in shares) at Mar. 31, 2019	100	100