Statements of Common Shareholder's Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Other Paid-In Capital	Retained Earnings
Beginning balance at Dec. 31, 2014	$ 597.0	$ 23.0	$ 265.4	$ 308.6
Beginning balance (in shares) at Dec. 31, 2014		9,190
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64.3			64.3
Dividends	(38.0)			(38.0)
Share-based compensation	1.4		1.4
Ending balance at Dec. 31, 2015	624.7	$ 23.0	266.8	334.9
Ending balance (in shares) at Dec. 31, 2015		9,190
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	57.2			57.2
Equity contribution from parent	2.7		2.7
Dividends	(30.0)			(30.0)
Share-based compensation	3.0		3.0
Ending balance at Dec. 31, 2016	657.6	$ 23.0	272.5	362.1
Ending balance (in shares) at Dec. 31, 2016		9,190
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	67.5			67.5
Ending balance at Dec. 31, 2017	$ 725.1	$ 23.0	$ 272.5	$ 429.6
Ending balance (in shares) at Dec. 31, 2017		9,190