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INVESTMENTS. (Unrealized Losses on Available-For-Sale T. Rowe Price Investment Portfolios) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions [Abstract]    
Number of holdings with unrealized holding losses less than 12 months (number of securities) | security 2 8
Number of holdings with unrealized holding losses 12 months or more (number of securities) | security 2 2
Total number of holdings with unrealized holding losses (number of securities) | security 4 10
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]    
Unrealized holding losses of holdings that were in a continuous loss for less than 12 months $ (0.4) $ (4.2)
Unrealized holding losses of holdings that were in a continuous loss for 12 months or more (4.1) (2.7)
Total unrealized holding losses of holdings that were in a continuous loss (4.5) (6.9)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Aggregate fair value of holdings with unrealized holding losses for less than 12 months 213.2 328.1
Aggregate fair value of holdings with unrealized holding losses for 12 months or more 191.5 169.5
Total aggregate fair value of holdings with unrealized holding losses $ 404.7 $ 497.6