Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,499,093	$ 1,451,526
Percentage of investments in unrealized loss position	100.00%
Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	84.80%
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	7.50%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	7.70%
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,275,871	1,240,264
Percentage of investments in unrealized loss position	100.00%
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 107,355	23,246
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 146,655	157,380
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 13,868	5,653
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 209,272	155,415
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,309,321	684,635
Percentage of investments in unrealized loss position	100.00%
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 375,353	149,827
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 114,047	58,497
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,942,658
Percentage of investments in unrealized loss position	85.30%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 163,409
Percentage of investments in unrealized loss position	7.20%
Debt Securities | Investment Grade | U.S. Treasury Bonds | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 107,355
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | Municipal Bonds | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 136,568
Percentage of investments in unrealized loss position	93.20%
Debt Securities | Investment Grade | Municipal Bonds | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,040
Percentage of investments in unrealized loss position	3.40%
Debt Securities | Investment Grade | Foreign Government Bonds | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,824
Percentage of investments in unrealized loss position	42.00%
Debt Securities | Investment Grade | Foreign Government Bonds | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 7,105
Percentage of investments in unrealized loss position	51.20%
Debt Securities | Investment Grade | Governmental Agency Bonds | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 209,272
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | Governmental Agency Mortgage-Backed Securities | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,309,321
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | U.S. Corporate Debt Securities | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 134,224
Percentage of investments in unrealized loss position	35.80%
Debt Securities | Investment Grade | U.S. Corporate Debt Securities | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 112,373
Percentage of investments in unrealized loss position	29.90%
Debt Securities | Investment Grade | Foreign Corporate Debt Securities | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 40,094
Percentage of investments in unrealized loss position	35.20%
Debt Securities | Investment Grade | Foreign Corporate Debt Securities | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 38,891
Percentage of investments in unrealized loss position	34.10%
Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 169,804
Percentage of investments in unrealized loss position	7.50%
Debt Securities | Non-Investment Grade | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,047
Percentage of investments in unrealized loss position	3.40%
Debt Securities | Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 939
Percentage of investments in unrealized loss position	6.80%
Debt Securities | Non-Investment Grade | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 128,756
Percentage of investments in unrealized loss position	34.30%
Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 35,062
Percentage of investments in unrealized loss position	30.70%
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 223,222	$ 211,262
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 15,014
Percentage of investments in unrealized loss position	100.00%
Equity securities | Investment Grade | Preferred stocks | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 8,850
Percentage of investments in unrealized loss position	58.90%
Equity securities | Non-Investment Grade | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 6,164
Percentage of investments in unrealized loss position	41.10%
Available For Sale Securities Excluding Common Stock
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,290,885
Available For Sale Securities Excluding Common Stock | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	1,942,658
Available For Sale Securities Excluding Common Stock | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	172,259
Available For Sale Securities Excluding Common Stock | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 175,968