Carrying Value and Estimated Fair Value of Consolidated Financial Instruments (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Assets
Investments (excluding equity method investments and loans)	[1]	$ 17,284.1	$ 17,406.5
Liabilities
Senior Notes and other debt	[2]	1,677.8	1,614.6
Carrying Value
Assets
Investments (excluding equity method investments and loans)	[1]	17,284.1	17,406.5
Liabilities
Senior Notes and other debt	[2]	$ 1,540.9	$ 1,484.9

[1] This table includes debt and equity securities, as well as partnership and non-marketable equity investments carried at fair value that are included in other invested assets. This table excludes investments accounted for using the equity method and commercial mortgage loans that are carried at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is discussed below.

[2] See Note 8 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, "Financial Statements and Supplementary Data" of the 2017 Form 10-K for additional information on the senior notes and other debt.