Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 4,592,170	$ 4,279,347
Equity securities	$ 387,921	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	$ 3,935,121
Percentage of investment portfolio by credit rating agencies	85.40%
Investment Grade | A- Rating or Higher | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	85.70%
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 367,804
Percentage of investment portfolio by credit rating agencies	8.20%
Investment Grade | BBB+ to BBB- Ratings | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	8.00%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 289,245
Percentage of investment portfolio by credit rating agencies	6.40%
Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	6.30%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 150,434	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	$ 150,434
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%
U.S. Treasury Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
U.S. Treasury 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	0.00%
U.S. Treasury Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
U.S. Treasury Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.00%
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 924,237	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	$ 879,726
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	95.20%
Municipal Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 35,861
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	3.80%
Municipal Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 8,650
Municipal Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	1.00%
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 145,358	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	$ 121,153
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	83.40%
Foreign Government Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 22,003
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	15.10%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,202
Foreign Government Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	1.50%
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 195,177	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	$ 195,177
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 Bonds | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Governmental Agency 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	0.00%
Governmental Agency Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Governmental Agency Bonds | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.00%
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 2,235,797	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	$ 2,235,797
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%
Governmental Agency Mortgage-Backed Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Governmental Agency Mortgage-Backed 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	0.00%
Governmental Agency Mortgage-Backed Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Governmental Agency Mortgage-Backed Securities | Non-Investment Grade | Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.00%
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 690,738	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	$ 237,952
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	34.50%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 222,588
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	32.20%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 230,198
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	33.30%
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 250,429	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	$ 114,882
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	45.90%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 87,352
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	34.90%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	$ 48,195
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	19.20%
Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 15,219	$ 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 | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 0
Preferred stocks | Investment Grade | A- Rating or Higher | Equity securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	0.00%
Preferred stocks | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 8,987
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	59.10%
Preferred stocks | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Equity securities	$ 6,232
Preferred stocks | Non-Investment Grade | Equity securities
Schedule Of Available For Sale Securities [Line Items]
Percentage of investment portfolio by credit rating agencies	40.90%
Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 4,607,389
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	3,935,121
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	376,791
Available For Sale Securities Excluding Common Stocks | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Total	$ 295,477