Gross Unrealized Losses and Related Fair Values for AFS Securities Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		$ 2,749.1
Securities, less than 12 months, gross unrealized losses		24.5
Securities, 12 months or more, fair value		1,354.7
Securities, 12 months or more, gross unrealized losses		32.6
Total, fair value		4,103.8
Total, gross unrealized losses		57.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		3.8
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value		145.7
Securities, less than 12 months, gross unrealized losses		3.8
Total, fair value		145.7
Total, gross unrealized losses		3.8
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 4,668.7	2,603.4
Securities, less than 12 months, gross unrealized losses	78.1	20.7
Securities, 12 months or more, fair value	1,206.7	1,354.7
Securities, 12 months or more, gross unrealized losses	51.6	32.6
Total, fair value	5,875.4	3,958.1
Total, gross unrealized losses	129.7	53.3
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	480.1	447.8
Securities, less than 12 months, gross unrealized losses	10.8	4.4
Securities, 12 months or more, fair value	381.6	416.6
Securities, 12 months or more, gross unrealized losses	18.9	13.2
Total, fair value	861.7	864.4
Total, gross unrealized losses	29.7	17.6
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	594.7	240.0
Securities, less than 12 months, gross unrealized losses	9.5	1.5
Securities, 12 months or more, fair value	220.5	267.3
Securities, 12 months or more, gross unrealized losses	9.3	5.1
Total, fair value	815.2	507.3
Total, gross unrealized losses	18.8	6.6
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	301.5	321.9
Securities, less than 12 months, gross unrealized losses	3.9	2.7
Securities, 12 months or more, fair value	70.6	72.2
Securities, 12 months or more, gross unrealized losses	2.2	2.0
Total, fair value	372.1	394.1
Total, gross unrealized losses	6.1	4.7
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,070.4	568.8
Securities, less than 12 months, gross unrealized losses	21.1	6.1
Securities, 12 months or more, fair value	191.4	207.3
Securities, 12 months or more, gross unrealized losses	8.1	3.6
Total, fair value	1,261.8	776.1
Total, gross unrealized losses	29.2	9.7
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	648.3	417.4
Securities, less than 12 months, gross unrealized losses	10.2	3.0
Securities, 12 months or more, fair value	155.4	159.4
Securities, 12 months or more, gross unrealized losses	5.4	3.5
Total, fair value	803.7	576.8
Total, gross unrealized losses	15.6	6.5
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	768.6	284.2
Securities, less than 12 months, gross unrealized losses	16.7	1.6
Securities, 12 months or more, fair value	120.5	131.5
Securities, 12 months or more, gross unrealized losses	5.0	3.0
Total, fair value	889.1	415.7
Total, gross unrealized losses	21.7	4.6
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	223.7	112.2
Securities, less than 12 months, gross unrealized losses	3.0	0.5
Securities, 12 months or more, fair value	32.7	34.7
Securities, 12 months or more, gross unrealized losses	2.2	1.8
Total, fair value	256.4	146.9
Total, gross unrealized losses	5.2	2.3
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	581.4	211.1
Securities, less than 12 months, gross unrealized losses	2.9	0.9
Securities, 12 months or more, fair value	34.0	65.7
Securities, 12 months or more, gross unrealized losses	0.5	0.4
Total, fair value	615.4	276.8
Total, gross unrealized losses	$ 3.4	$ 1.3