Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4.2	$ 2.4
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Included in net income (loss)	(0.2)	0.0
Included in other comprehensive income (loss)	(0.1)	0.1
Purchases	10.6	1.7
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Ending balance	14.5	4.2
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	0.0
Credit-Financial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1.9	2.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Included in net income (loss)	0.0	0.0
Included in other comprehensive income (loss)	(0.1)	(0.1)
Purchases	2.2	0.0
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Ending balance	4.0	1.9
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	0.0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2.3	0.4
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Included in net income (loss)	(0.2)	0.0
Included in other comprehensive income (loss)	0.0	0.2
Purchases	8.4	1.7
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Ending balance	10.5	2.3
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	$ 0.0	$ 0.0