Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (Available-for-Sale Securities [Member], Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Available-for-Sale Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 9.4	$ 8.9
Total realized gains (losses) - included in earnings	0.2	0
Total unrealized gains (losses) - included in other comprehensive income	(0.2)	0.2
Purchases	0	0.3
Sales	(5.9)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 3.5	$ 9.4