Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment
Mortgage Loans	$ 2,145.3	$ 2,038.9
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	0.4	0.7
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	482.8	488.2
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,625.5	1,506.6
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	36.6	43.4
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	967.2	917.9
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	1,080.1	1,011.5
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	46.1	50.8
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 51.9	$ 58.7