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Investments - Schedule of Analysis of Investment Operations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Investment Securities [Line Items]                      
Investment income, gross                 $ 744,969 $ 725,755 $ 716,556
Less investment expense                 (35,226) (32,111) (23,528)
Net investment income 178,284 176,656 177,964 176,839 174,947 169,400 175,176 174,121 709,743 693,644 693,028
Long-term Debt [Member]
                     
Investment Securities [Line Items]                      
Investment income, gross                 1,281 2,320 2,439
Short-term Debt [Member]
                     
Investment Securities [Line Items]                      
Investment income, gross                 138 311 165
Fixed Maturities [Member]
                     
Investment Securities [Line Items]                      
Investment income, gross                 709,756 691,229 683,101
Equity Securities [Member]
                     
Investment Securities [Line Items]                      
Investment income, gross                 323 1,178 1,558
Policy Loans [Member]
                     
Investment Securities [Line Items]                      
Investment income, gross                 $ 33,471 $ 30,717 $ 29,293