Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,359,497	$ 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	$ 1,219,083
Investment Grade | A- Rating or Higher
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	81.30%
Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	8.30%
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	$ 990,592
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	$ 101,450
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Percentage of investments in unrealized loss position	10.40%
Non-Investment Grade | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 127,041
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,204,056	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	$ 37,870	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	$ 74,614	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	$ 14,245	13,868
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	$ 37,002	209,272
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	$ 791,949	1,309,321
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	$ 185,333	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	$ 63,043	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	$ 990,592
Percentage of investments in unrealized loss position	82.30%
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	$ 37,870
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	$ 64,560
Percentage of investments in unrealized loss position	86.60%
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	$ 7,567
Percentage of investments in unrealized loss position	53.10%
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	$ 37,002
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	$ 791,949
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	$ 37,102
Percentage of investments in unrealized loss position	20.10%
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	$ 14,542
Percentage of investments in unrealized loss position	23.10%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 92,581
Percentage of investments in unrealized loss position	7.70%
Debt Securities | Investment Grade | BBB+ to BBB- Ratings | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,034
Percentage of investments in unrealized loss position	6.70%
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	$ 5,566
Percentage of investments in unrealized loss position	39.10%
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	$ 59,762
Percentage of investments in unrealized loss position	32.20%
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	$ 22,219
Percentage of investments in unrealized loss position	35.20%
Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 120,883
Percentage of investments in unrealized loss position	10.00%
Debt Securities | Non-Investment Grade | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 5,020
Percentage of investments in unrealized loss position	6.70%
Debt Securities | Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,112
Percentage of investments in unrealized loss position	7.80%
Debt Securities | Non-Investment Grade | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 88,469
Percentage of investments in unrealized loss position	47.70%
Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 26,282
Percentage of investments in unrealized loss position	41.70%
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 155,441	$ 223,222
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 15,027
Percentage of investments in unrealized loss position	100.00%
Equity securities | Investment Grade | BBB+ to BBB- Ratings | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 8,869
Percentage of investments in unrealized loss position	59.00%
Equity securities | Non-Investment Grade | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 6,158
Percentage of investments in unrealized loss position	41.00%