Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,258,945	$ 4,279,347
Equity securities	$ 396,637	321,285
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 3,755,896
Percentage of investment portfolio by credit rating agencies	87.90%
Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	88.20%
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 273,941
Percentage of investment portfolio by credit rating agencies	6.60%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	6.40%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 229,108
Percentage of investment portfolio by credit rating agencies	5.50%
Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	5.40%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 144,394	129,372
U.S. Treasury Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
U.S. Treasury Bonds | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 144,394
U.S. Treasury Bonds | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 699,524	703,795
Municipal Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Municipal Bonds | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 670,617
Municipal Bonds | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	95.90%
Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 23,887
Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	3.40%
Municipal Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 5,020
Municipal Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.70%
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 129,577	130,101
Foreign Government Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Foreign Government Bonds | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 120,950
Foreign Government Bonds | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	93.30%
Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,515
Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	5.80%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 1,112
Foreign Government Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.90%
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 354,941	418,091
Governmental Agency Bonds | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Governmental Agency Bonds | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 354,941
Governmental Agency Bonds | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,044,958	2,055,673
Governmental Agency Mortgage-Backed Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,044,958
Governmental Agency Mortgage-Backed Securities | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 659,865	634,683
U.S. Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
U.S. Corporate Debt Securities | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 296,764
U.S. Corporate Debt Securities | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	45.00%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 182,015
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	27.60%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 181,086
U.S. Corporate Debt Securities | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	27.40%
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 225,686	207,632
Foreign Corporate Debt Securities | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Foreign Corporate Debt Securities | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 123,272
Foreign Corporate Debt Securities | Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	54.60%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 60,524
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	26.80%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 41,890
Foreign Corporate Debt Securities | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	18.60%
Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 15,446	$ 15,467
Preferred stocks | Equity securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	100.00%
Preferred stocks | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 8,869
Preferred stocks | Investment Grade | BBB+ to BBB- Ratings | Equity securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	57.40%
Preferred stocks | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 6,577
Preferred stocks | Non-Investment Grade | Equity securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	42.60%
Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 4,274,391
Available For Sale Securities Excluding Common Stocks | Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	3,755,896
Available For Sale Securities Excluding Common Stocks | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	282,810
Available For Sale Securities Excluding Common Stocks | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 235,685