STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning Balance at Dec. 31, 2015	$ 1,807,900	$ 0	$ 1,371,200	$ 438,000	$ (1,300)
Beginning Balance (in shares) at Dec. 31, 2015		100
Comprehensive income:
Net income	152,200			152,200
Other comprehensive income	0
Common dividends declared to parent	(103,500)			103,500
Contribution of capital by parent	75,000		75,000
Ending Balance at Dec. 31, 2016	1,931,600	$ 0	1,446,200	486,700	(1,300)
Ending Balance (in shares) at Dec. 31, 2016		100
Comprehensive income:
Net income	159,200			159,200
Other comprehensive income	100				100
Common dividends declared to parent	(104,600)			104,600
Contribution of capital by parent	144,000		144,000
Adoption of ASU No. 2018-02	0			0	(300)
Ending Balance at Dec. 31, 2017	$ 2,130,300	$ 0	1,590,200	541,600	(1,500)
Ending Balance (in shares) at Dec. 31, 2017	100	100
Comprehensive income:
Net income	$ 213,300			213,300
Other comprehensive income	100				100
Common dividends declared to parent	(149,200)			149,200
Contribution of capital by parent	342,100		342,100
Ending Balance at Dec. 31, 2018	$ 2,536,600	$ 0	$ 1,932,300	$ 605,700	$ (1,400)
Ending Balance (in shares) at Dec. 31, 2018	100	100