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Investments (Details)
$ in Thousands
Sep. 30, 2015
USD ($)
security
Dec. 31, 2014
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 4,603,942 $ 4,605,517
Gross Unrealized Gains 38,912 37,649
Gross Unrealized Losses [1] (48,889) (30,497)
Fair Value $ 4,593,965 4,612,669
Percentage of foreign investments held in government and semi-government securities (in hundredths) 86.00%  
Percentage of foreign investments held in corporate securities (in hundredths) 10.00%  
Percentage of foreign investments held in cash equivalents (in hundredths) 4.00%  
Percentage of Australian portfolio rated AAA (in hundredths) 87.00%  
Percentage of Australian portfolio rated AA (in hundredths) 13.00%  
Gross unrealized (gains) losses $ 48,900 30,500
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 1,456,594 1,209,256
12 months or greater 547,266 1,035,884
Total investment portfolio 2,003,860 2,245,140
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 35,353 7,388
12 months or greater 13,536 23,109
Total investment portfolio $ 48,889 $ 30,497
Number of securities in unrealized loss position | security 369 423
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 104,419 $ 349,153
Gross Unrealized Gains 2,287 2,752
Gross Unrealized Losses [1] (1,740) (5,130)
Fair Value 104,966 346,775
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 25,781 58,166
12 months or greater 15,534 232,351
Total investment portfolio 41,315 290,517
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 295 138
12 months or greater 1,445 4,992
Total investment portfolio 1,740 5,130
Obligations of U.S. states and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,595,458 844,942
Gross Unrealized Gains 24,306 12,961
Gross Unrealized Losses [1] (7,243) (2,761)
Fair Value 1,612,521 855,142
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 231,647 166,408
12 months or greater 43,865 114,465
Total investment portfolio 275,512 280,873
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 6,626 1,066
12 months or greater 617 1,695
Total investment portfolio 7,243 2,761
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 2,142,500 2,418,991
Gross Unrealized Gains 7,722 16,325
Gross Unrealized Losses [1] (29,650) (10,035)
Fair Value 2,120,572 2,425,281
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 1,045,477 816,555
12 months or greater 140,995 243,208
Total investment portfolio 1,186,472 1,059,763
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 27,190 5,259
12 months or greater 2,460 4,776
Total investment portfolio 29,650 10,035
Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 132,271 286,260
Gross Unrealized Gains 346 535
Gross Unrealized Losses [1] (80) (140)
Fair Value 132,537 286,655
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 36,920 54,491
12 months or greater 5,567 11,895
Total investment portfolio 42,487 66,386
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 64 80
12 months or greater 16 60
Total investment portfolio 80 140
Residential mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 278,737 329,983
Gross Unrealized Gains 275 254
Gross Unrealized Losses [1] (7,541) (9,000)
Fair Value 271,471 321,237
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 39,870 24,168
12 months or greater 201,435 263,002
Total investment portfolio 241,305 287,170
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 414 34
12 months or greater 7,127 8,966
Total investment portfolio 7,541 9,000
Commercial mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 253,143 276,215
Gross Unrealized Gains 1,048 1,221
Gross Unrealized Losses [1] (1,504) (2,158)
Fair Value 252,687 275,278
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 74,403 89,301
12 months or greater 79,411 110,652
Total investment portfolio 153,814 199,953
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 698 810
12 months or greater 806 1,348
Total investment portfolio 1,504 2,158
Collateralized loan obligations    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 61,342 61,340
Gross Unrealized Gains 0 0
Gross Unrealized Losses [1] (1,056) (1,264)
Fair Value 60,286 60,076
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 0 0
12 months or greater 60,286 60,076
Total investment portfolio 60,286 60,076
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 0 0
12 months or greater 1,056 1,264
Total investment portfolio 1,056 1,264
Debt securities issued by foreign sovereign governments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 30,477 35,630
Gross Unrealized Gains 2,876 3,540
Gross Unrealized Losses [1] (62) 0
Fair Value 33,291 39,170
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 2,177  
12 months or greater 0  
Total investment portfolio 2,177  
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 62  
12 months or greater 0  
Total investment portfolio 62  
Total debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,598,347 4,602,514
Gross Unrealized Gains 38,860 37,588
Gross Unrealized Losses [1] (48,876) (30,488)
Fair Value 4,588,331 4,609,614
Equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 5,595 3,003
Gross Unrealized Gains 52 61
Gross Unrealized Losses [1] (13) (9)
Fair Value 5,634 3,055
Continuous Unrealized Loss Portion, Fair Value [Abstract]    
Less than 12 months 319 167
12 months or greater 173 235
Total investment portfolio 492 402
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]    
Less than 12 months 4 1
12 months or greater 9 8
Total investment portfolio $ 13 $ 9
[1] At September 30, 2015 and December 31, 2014, there were no other-than-temporary impairment losses recorded in other comprehensive income.