Investments - Credit Ratings of Fixed Maturity and Short-Term Investments, Trading (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($)
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,400,332
Fair Value	5,411,691
Amortized Cost	6,170,987
Fair Value	6,219,870
Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	227,077
Total fixed maturity and short-term investments	$ 227,077
Marketable Securities Member | Portfolio Concentration Risk Member
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	100.00%
Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	87.00%
Marketable Securities Member | Credit Rating Concentration Risk | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	3.70%
Trading and Available for Sale Securities | Credit Rating Concentration Risk | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	4.20%
AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,889,882
Total fixed maturity and short-term investments	$ 1,951,173
AAA Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	31.40%
AAA Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	34.90%
AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 815,272
Total fixed maturity and short-term investments	$ 940,110
AA Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	15.10%
AA Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	15.10%
A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,630,124
Total fixed maturity and short-term investments	$ 2,143,533
A Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	34.40%
A Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	30.00%
BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 803,186
Total fixed maturity and short-term investments	$ 911,708
BBB Rated | Marketable Securities Member | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	14.70%
BBB Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	14.90%
Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 46,150
Total fixed maturity and short-term investments	$ 46,269
Not Rated | Marketable Securities Member | Credit Rating Concentration Risk
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	0.70%
Not Rated | Trading and Available for Sale Securities | Credit Rating Concentration Risk
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	0.90%
U.S. government and agency
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 771,905
Fair Value	785,017
U.S. government and agency | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
U.S. government and agency | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	12.60%
U.S. government and agency | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 778,456
U.S. government and agency | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	6,561
U.S. government and agency | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.S. government and agency | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
U.S. government and agency | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Non-U.S. government
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	405,688
Fair Value	400,001
Non-U.S. government | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 11,306
Non-U.S. government | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	6.40%
Non-U.S. government | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 129,956
Non-U.S. government | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	185,750
Non-U.S. government | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	52,705
Non-U.S. government | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	20,284
Non-U.S. government | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Corporate
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	2,777,899
Fair Value	2,798,972
Corporate | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 143,010
Corporate | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	45.10%
Corporate | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 164,171
Corporate | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	451,121
Corporate | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	1,312,900
Corporate | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	721,700
Corporate | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	6,070
Municipal
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	14,987
Fair Value	15,293
Municipal | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 0
Municipal | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	0.20%
Municipal | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,395
Municipal | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	7,710
Municipal | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	2,188
Municipal | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Municipal | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	0
Residential mortgage-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	474,351
Fair Value	475,444
Residential mortgage-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,036
Residential mortgage-backed | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	7.60%
Residential mortgage-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 465,622
Residential mortgage-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	452
Residential mortgage-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	6,029
Residential mortgage-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	2,302
Residential mortgage-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	3
Commercial mortgage-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	280,949
Fair Value	281,052
Commercial mortgage-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,674
Commercial mortgage-backed | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	4.50%
Commercial mortgage-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 114,235
Commercial mortgage-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	34,730
Commercial mortgage-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	73,995
Commercial mortgage-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	15,538
Commercial mortgage-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	39,880
Asset-backed
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	674,553
Fair Value	655,912
Asset-backed | Non-Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 69,051
Asset-backed | Marketable Securities Member | Portfolio Concentration Risk Member | Trading Securities and Available for Sale Securities
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
% of Total Investments	10.60%
Asset-backed | AAA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 232,047
Asset-backed | AA Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	128,948
Asset-backed | A Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	182,307
Asset-backed | BBB Rated | Investment Grade
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	43,362
Asset-backed | Not Rated
Schedule of Trading Securities and Available-for-sale Securities [Line Items]
Amortized Cost	$ 197