Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,689,425	$ 3,137,507
Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,614,913
Percentage of investments in unrealized loss position	100.00%
Investment Grade | Available For Sale Securities Excluding Common Stocks | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,387,849
Percentage of investments in unrealized loss position	91.30%
Investment Grade | Available For Sale Securities Excluding Common Stocks | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 158,618
Percentage of investments in unrealized loss position	6.10%
Non-Investment Grade | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 68,446
Percentage of investments in unrealized loss position	2.60%
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,597,614	3,008,227
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	$ 101,603	111,748
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	$ 521,687	652,016
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	$ 35,682	63,368
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	$ 148,314	148,112
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	$ 1,554,751	1,728,139
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	$ 185,321	218,032
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	$ 50,256	86,812
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	$ 2,387,849
Percentage of investments in unrealized loss position	91.90%
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	$ 101,603
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	$ 479,317
Percentage of investments in unrealized loss position	91.80%
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	$ 19,233
Percentage of investments in unrealized loss position	53.90%
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	$ 148,314
Percentage of investments in unrealized loss position	100.00%
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	$ 1,554,751
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	$ 64,826
Percentage of investments in unrealized loss position	35.00%
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	$ 19,805
Percentage of investments in unrealized loss position	39.40%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 148,570
Percentage of investments in unrealized loss position	5.70%
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	$ 37,368
Percentage of investments in unrealized loss position	7.20%
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	$ 13,010
Percentage of investments in unrealized loss position	36.50%
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 | 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 | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 75,090
Percentage of investments in unrealized loss position	40.50%
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	$ 23,102
Percentage of investments in unrealized loss position	46.00%
Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 61,195
Percentage of investments in unrealized loss position	2.40%
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	$ 5,002
Percentage of investments in unrealized loss position	1.00%
Debt Securities | Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 3,439
Percentage of investments in unrealized loss position	9.60%
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 | 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 | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 45,405
Percentage of investments in unrealized loss position	24.50%
Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 7,349
Percentage of investments in unrealized loss position	14.60%
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 91,811	$ 129,280
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 17,299
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	$ 10,048
Percentage of investments in unrealized loss position	58.10%
Equity securities | Non-Investment Grade | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 7,251
Percentage of investments in unrealized loss position	41.90%