Debt and Equity Securities (Composition of Investment Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,562,232	$ 3,137,507
Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,509,763
Percentage of investments in unrealized loss position	100.00%
Investment Grade | Available For Sale Securities Excluding Common Stocks | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,326,268
Percentage of investments in unrealized loss position	92.70%
Investment Grade | Available For Sale Securities Excluding Common Stocks | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 136,621
Percentage of investments in unrealized loss position	5.40%
Non-Investment Grade | Available For Sale Securities Excluding Common Stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 46,874
Percentage of investments in unrealized loss position	1.90%
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,497,710	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	$ 81,677	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	$ 371,977	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	$ 107,812	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	$ 187,836	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	$ 1,519,065	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	$ 163,180	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	$ 66,163	86,812
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,326,268
Percentage of investments in unrealized loss position	93.10%
Debt Securities | Investment Grade | A- or Higher | U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 81,677
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- or Higher | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 355,301
Percentage of investments in unrealized loss position	95.60%
Debt Securities | Investment Grade | A- or Higher | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 95,300
Percentage of investments in unrealized loss position	88.40%
Debt Securities | Investment Grade | A- or Higher | Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 187,836
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- or Higher | Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 1,519,065
Percentage of investments in unrealized loss position	100.00%
Debt Securities | Investment Grade | A- or Higher | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 46,440
Percentage of investments in unrealized loss position	28.50%
Debt Securities | Investment Grade | A- or Higher | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 40,649
Percentage of investments in unrealized loss position	61.40%
Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 128,631
Percentage of investments in unrealized loss position	5.10%
Debt Securities | Investment Grade | BBB+ to BBB- | 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- | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 15,057
Percentage of investments in unrealized loss position	4.00%
Debt Securities | Investment Grade | BBB+ to BBB- | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 9,663
Percentage of investments in unrealized loss position	9.00%
Debt Securities | Investment Grade | BBB+ to BBB- | 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- | 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- | U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 80,803
Percentage of investments in unrealized loss position	49.50%
Debt Securities | Investment Grade | BBB+ to BBB- | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 23,108
Percentage of investments in unrealized loss position	34.90%
Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 42,811
Percentage of investments in unrealized loss position	1.80%
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	$ 1,619
Percentage of investments in unrealized loss position	0.40%
Debt Securities | Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,849
Percentage of investments in unrealized loss position	2.60%
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	$ 35,937
Percentage of investments in unrealized loss position	22.00%
Debt Securities | Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 2,406
Percentage of investments in unrealized loss position	3.70%
Equity securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 64,522	$ 129,280
Equity securities | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 12,053
Percentage of investments in unrealized loss position	100.00%
Equity securities | Investment Grade | A- 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- | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 7,990
Percentage of investments in unrealized loss position	66.30%
Equity securities | Non-Investment Grade | Preferred stocks
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 4,063
Percentage of investments in unrealized loss position	33.70%