Composition of Investments (Detail) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Debt Securities
Available For Sale Securities Amortized Cost	$ 6,079,982,000	$ 5,337,845,000
Available For Sale Securities Unrealized Gains	39,060,000	54,059,000
Available For Sale Securities Unrealized Losses	(46,769,000)	(19,811,000)
Available For Sale Securities Fair Value	6,072,273,000	5,372,093,000
Debt Securities | US Government Agencies Debt Securities
Available For Sale Securities Amortized Cost	2,294,101,000	1,737,782,000
Available For Sale Securities Unrealized Gains	14,889,000	11,286,000
Available For Sale Securities Unrealized Losses	(2,268,000)	(2,674,000)
Available For Sale Securities Fair Value	2,306,722,000	1,746,394,000
Debt Securities | US Corporate Debt Securities
Available For Sale Securities Amortized Cost	1,391,063,000	1,239,325,000
Available For Sale Securities Unrealized Gains	10,175,000	13,608,000
Available For Sale Securities Unrealized Losses	(3,595,000)	(1,307,000)
Available For Sale Securities Fair Value	1,397,643,000	1,251,626,000
Debt Securities | High Yield Corporate Debt Securities
Available For Sale Securities Amortized Cost	448,200,000	421,469,000
Available For Sale Securities Unrealized Gains	4,549,000	18,306,000
Available For Sale Securities Unrealized Losses	(27,557,000)	(1,943,000)
Available For Sale Securities Fair Value	425,192,000	437,832,000
Debt Securities | Asset-backed Securities
Available For Sale Securities Amortized Cost	31,411,000	34,730,000
Available For Sale Securities Unrealized Gains	103,000	152,000
Available For Sale Securities Unrealized Losses	(11,000)	(1,000)
Available For Sale Securities Fair Value	31,503,000	34,881,000
Debt Securities | Municipal Obligations
Available For Sale Securities Amortized Cost	775,608,000	1,095,338,000
Available For Sale Securities Unrealized Gains	1,953,000	3,829,000
Available For Sale Securities Unrealized Losses	(479,000)	(3,266,000)
Available For Sale Securities Fair Value	777,082,000	1,095,901,000
Debt Securities | Auction Rate Securities
Available For Sale Securities Amortized Cost	91,350,000	155,950,000
Available For Sale Securities Unrealized Gains	0	0
Available For Sale Securities Unrealized Losses	(10,259,000)	(9,906,000)
Available For Sale Securities Fair Value	81,091,000	146,044,000
Debt Securities | Foreign Debt Securities
Available For Sale Securities Amortized Cost	1,048,249,000	653,251,000
Available For Sale Securities Unrealized Gains	7,391,000	6,878,000
Available For Sale Securities Unrealized Losses	(2,600,000)	(714,000)
Available For Sale Securities Fair Value	1,053,040,000	659,415,000
Debt and Equity Securities
Available For Sale Securities Amortized Cost	6,138,187,000	5,360,221,000
Available For Sale Securities Unrealized Gains	43,488,000	86,507,000
Available For Sale Securities Unrealized Losses	(46,769,000)	(19,811,000)
Available For Sale Securities Fair Value	6,134,906,000	5,426,917,000
Debt and Equity Securities | Publicly Traded Equity Securities
Available For Sale Securities Amortized Cost	58,205,000	22,376,000
Available For Sale Securities Unrealized Gains	4,428,000	32,448,000
Available For Sale Securities Unrealized Losses	0	0
Available For Sale Securities Fair Value	$ 62,633,000	$ 54,824,000