Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 3,137,507	$ 2,499,093
Percentage of investments in unrealized loss position	100.00%
Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 3,023,809
Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	91.10%
Investment Grade | A- Rating or Higher | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,756,093
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	6.60%
Investment Grade | BBB+ to BBB- Ratings | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 198,410
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	2.30%
Non-Investment Grade | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 69,306
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 3,008,227	2,275,871
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	$ 111,748	107,355
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	$ 652,016	146,655
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	$ 63,368	13,868
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	$ 1,728,139	1,309,321
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	$ 148,112	209,272
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	$ 218,032	375,353
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	$ 86,812	114,047
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	$ 2,756,093
Percentage of investments in unrealized loss position	91.60%
Debt Securities | Investment Grade | A- Rating or Higher | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 111,748
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- Rating or Higher | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 609,018
Percentage of investments in unrealized loss position	93.40%
Debt Securities | Investment Grade | A- Rating or Higher | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 43,123
Percentage of investments in unrealized loss position	68.00%
Debt Securities | Investment Grade | A- Rating or Higher | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,728,139
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- Rating or Higher | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 148,112
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- Rating or Higher | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 79,166
Percentage of investments in unrealized loss position	36.30%
Debt Securities | Investment Grade | A- Rating or Higher | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 36,787
Percentage of investments in unrealized loss position	42.40%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 189,207
Percentage of investments in unrealized loss position	6.30%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 37,388
Percentage of investments in unrealized loss position	5.70%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 18,290
Percentage of investments in unrealized loss position	28.90%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 95,320
Percentage of investments in unrealized loss position	43.70%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 38,209
Percentage of investments in unrealized loss position	44.00%
Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 62,927
Percentage of investments in unrealized loss position	2.10%
Debt Securities | Non-Investment Grade | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Non-Investment Grade | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,610
Percentage of investments in unrealized loss position	0.90%
Debt Securities | Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,955
Percentage of investments in unrealized loss position	3.10%
Debt Securities | Non-Investment Grade | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Non-Investment Grade | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Debt Securities | Non-Investment Grade | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 43,546
Percentage of investments in unrealized loss position	20.00%
Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 11,816
Percentage of investments in unrealized loss position	13.60%
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 129,280	$ 223,222
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 15,582
Percentage of investments in unrealized loss position	100.00%
Equity securities | Investment Grade | A- Rating or Higher | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 0
Percentage of investments in unrealized loss position	0.00%
Equity securities | Investment Grade | BBB+ to BBB- Ratings | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 9,203
Percentage of investments in unrealized loss position	59.10%
Equity securities | Non-Investment Grade | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 6,379
Percentage of investments in unrealized loss position	40.90%