CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2018		100
Beginning balance at Dec. 31, 2018	$ 6,298,200	$ 0	$ 4,340,500	$ 1,983,200	$ (25,500)
Increase (Decrease) in Stockholder's Equity
Net income	138,800			138,800
Other comprehensive income	300				300
Dividends declared to parent	(98,800)			(98,800)
Contribution of capital by parent	50,000		50,000
Balance (in shares) at Mar. 31, 2019		100
Ending balance at Mar. 31, 2019	$ 6,388,500	$ 0	4,390,500	2,023,200	(25,200)
Balance (in shares) at Dec. 31, 2019	100	100
Beginning balance at Dec. 31, 2019	$ 6,996,200	$ 0	4,939,400	2,083,400	(26,600)
Increase (Decrease) in Stockholder's Equity
Net income	128,700			128,700
Other comprehensive income	300				300
Dividends declared to parent	(102,500)			(102,500)
Contribution of capital by parent	$ 50,000		50,000
Balance (in shares) at Mar. 31, 2020	100	100
Increase (Decrease) in Stockholder's Equity
Adoption of ASC Topic 326	$ (700)			(700)
Ending balance at Mar. 31, 2020	$ 7,072,000	$ 0	$ 4,989,400	$ 2,108,900	$ (26,300)