Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 47.9	$ 61.4	$ 32.5	$ 37.6
Other comprehensive income before reclassifications	15.4	24.9	35.5	48.9
Amounts reclassified from AOCI	(9.9)	(3.6)	(14.6)	(3.8)
Other comprehensive income	5.5	21.3	20.9	45.1
Ending balance	53.4	82.7	53.4	82.7
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	77.8	91.7	62.8	68.5
Other comprehensive income before reclassifications	15.4	24.9	35.5	48.9
Amounts reclassified from AOCI	(10.2)	(4.3)	(15.3)	(5.1)
Other comprehensive income	5.2	20.6	20.2	43.8
Ending balance	83.0	112.3	83.0	112.3
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(29.9)	(30.3)	(30.3)	(30.9)
Other comprehensive income before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI	0.3	0.7	0.7	1.3
Other comprehensive income	0.3	0.7	0.7	1.3
Ending balance	$ (29.6)	$ (29.6)	$ (29.6)	$ (29.6)