Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment
Mortgage Loans	$ 2,206.6	$ 2,295.0
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	394.8	477.5
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,811.8	1,814.1
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	0.0	3.4
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	1,167.4	1,204.8
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	1,013.4	1,049.1
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	0.0	11.8
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 25.8	$ 29.3