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Net Investment Income (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | $ 615.3 |  | $ 614.8
Less Investment Expenses | 9.6 |  | 8.8
Less Investment Income on PFA Assets | 3.4 |  | 3.6
Net Investment Income | 602.3 |  | 602.4
Fixed Maturity Securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 557.1 |  | 563.7
Derivatives |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 15.4 |  | 13.4
Mortgage Loans |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 27.3 |  | 25.4
Policy Loans |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 4.4 |  | 4.3
Equity Securities |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | (0.2) | [1] | 0.3
Unrealized Loss on Securities | 2.0 |  |
Partnership Interest |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 5.6 | [1] | 5.1
Unrealized Loss on Securities | 1.6 |  |
Other Long-term Investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | 2.1 |  | 0.7
Short-term Investments |  |  |
Schedule of Investment Income, Reported Amounts, by Category |  |  |
Investment Income, Interest and Dividend | $ 3.6 |  | $ 1.9
[1] | 1 The net unrealized loss recognized in net investment income for the first three months of 2018 related to equity securities and private equity partnerships still held at March 31, 2018 was $2.0 million and $1.6 million, respectively.

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