Debt and Equity Securities (Composition of Investment Portfolio 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]
Debt securities, Estimated fair value	$ 4,279,347	$ 3,450,252
Equity securities	321,285	402,412
Estimated fair value, Total	$ 4,600,632	3,852,664
Percentage of investment portfolio by credit rating agencies	100.00%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 129,372	64,982
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	703,795	587,677
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	130,101	134,938
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	418,091	197,874
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	2,055,673	1,812,162
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	634,683	452,129
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	207,632	183,952
Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	15,467	$ 15,525
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,279,347
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 129,372
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 703,795
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 130,101
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 418,091
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,055,673
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 634,683
Percentage of investment portfolio by credit rating agencies	100.00%
Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 207,632
Percentage of investment portfolio by credit rating agencies	100.00%
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 15,467
Percentage of investment portfolio by credit rating agencies	100.00%
Available For Sale Securities Excluding Common Stock
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 4,294,814
Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	88.70%
Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 3,806,963
Percentage of investment portfolio by credit rating agencies	88.90%
Investment Grade | A- Rating or Higher | Debt Securities | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 129,372
Percentage of investment portfolio by credit rating agencies	100.00%
Investment Grade | A- Rating or Higher | Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 675,257
Percentage of investment portfolio by credit rating agencies	95.90%
Investment Grade | A- Rating or Higher | Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 121,975
Percentage of investment portfolio by credit rating agencies	93.80%
Investment Grade | A- Rating or Higher | Debt Securities | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 418,091
Percentage of investment portfolio by credit rating agencies	100.00%
Investment Grade | A- Rating or Higher | Debt Securities | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,055,673
Percentage of investment portfolio by credit rating agencies	100.00%
Investment Grade | A- Rating or Higher | Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 293,413
Percentage of investment portfolio by credit rating agencies	46.20%
Investment Grade | A- Rating or Higher | Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 113,182
Percentage of investment portfolio by credit rating agencies	54.50%
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	6.00%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 250,579
Percentage of investment portfolio by credit rating agencies	5.90%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 23,188
Percentage of investment portfolio by credit rating agencies	3.30%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 7,187
Percentage of investment portfolio by credit rating agencies	5.50%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 167,292
Percentage of investment portfolio by credit rating agencies	26.40%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 52,912
Percentage of investment portfolio by credit rating agencies	25.50%
Investment Grade | BBB+ to BBB- Ratings | Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 8,850
Percentage of investment portfolio by credit rating agencies	57.20%
Investment Grade | Available For Sale Securities Excluding Common Stock | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 3,806,963
Investment Grade | Available For Sale Securities Excluding Common Stock | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 259,429
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	5.30%
Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 221,805
Percentage of investment portfolio by credit rating agencies	5.20%
Non-Investment Grade | Debt Securities | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 5,350
Percentage of investment portfolio by credit rating agencies	0.80%
Non-Investment Grade | Debt Securities | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 939
Percentage of investment portfolio by credit rating agencies	0.70%
Non-Investment Grade | Debt Securities | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 173,978
Percentage of investment portfolio by credit rating agencies	27.40%
Non-Investment Grade | Debt Securities | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 41,538
Percentage of investment portfolio by credit rating agencies	20.00%
Non-Investment Grade | Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 6,617
Percentage of investment portfolio by credit rating agencies	42.80%
Non-Investment Grade | Available For Sale Securities Excluding Common Stock
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 228,422