SCHEDULE I - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities [Abstract]
Net cash provided by operating activities	$ 444.5	$ 200.5	$ 165.6
Cash Flows From Investing Activities [Abstract]
Net cash used in investing activities	(235.0)	(60.8)	(82.1)
Cash Flows From Financing Activities [Abstract]
Proceeds from stock option exercises	6.8	0.0	0.0
Purchases of treasury stock	(40.7)	(3.6)	(14.1)
Proceeds from the issuance of common stock	0.0	893.6	3.3
Net cash used in financing activities	(373.0)	(17.4)	(43.0)
(Decrease) increase in cash and cash equivalents	(172.1)	137.5	27.5
Cash and cash equivalents, beginning of year	393.3	255.8	228.3
Cash and cash equivalents, end of year	221.2	393.3	255.8
Parent Company [Member]
Cash Flows From Operating Activities [Abstract]
Net cash provided by operating activities	55.0	9.2	11.1
Cash Flows From Investing Activities [Abstract]
Advances to subsidiaries	(20.3)	(899.3)	0.0
Net cash used in investing activities	(20.3)	(899.3)	0.0
Cash Flows From Financing Activities [Abstract]
Proceeds from stock option exercises	6.8	0.0	0.0
Purchases of treasury stock	(40.7)	(3.6)	(14.1)
Proceeds from the issuance of common stock	0.0	893.6	3.3
Net cash used in financing activities	(33.9)	890.0	(10.8)
(Decrease) increase in cash and cash equivalents	0.8	(0.1)	0.3
Cash and cash equivalents, beginning of year	0.2	0.3	0.0
Cash and cash equivalents, end of year	$ 1.0	$ 0.2	$ 0.3