Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,839,530	$ 5,913,636
Equity securities, Estimated fair value	377,296	392,318
Estimated fair value, Total	$ 5,852,838
Percentage of debt securities by credit rating agencies	100.00%
Percentage of investment portfolio by credit rating agencies	100.00%
U.S. Treasury bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 122,708	143,941
Percentage of debt securities by credit rating agencies	100.00%
Municipal bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,048,942	1,090,839
Percentage of debt securities by credit rating agencies	100.00%
Foreign government bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 158,298	180,090
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 299,095	321,919
Percentage of debt securities by credit rating agencies	100.00%
Governmental agency mortgage-backed securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 3,348,376	3,278,258
Percentage of debt securities by credit rating agencies	100.00%
U.S. corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 539,281	553,372
Percentage of debt securities by credit rating agencies	100.00%
Foreign corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 322,830	345,217
Percentage of debt securities by credit rating agencies	100.00%
Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 13,308	$ 18,094
Percentage of equity securities by credit rating agencies	100.00%
Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,298,420
Estimated fair value, Total	$ 5,298,448
Percentage of debt securities by credit rating agencies	90.70%
Percentage of investment portfolio by credit rating agencies	90.50%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 441,038
Estimated fair value, Total	$ 453,627
Percentage of debt securities by credit rating agencies	7.60%
Percentage of investment portfolio by credit rating agencies	7.80%
Investment Grade | U.S. Treasury bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 122,708
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | Municipal bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,003,827
Percentage of debt securities by credit rating agencies	95.70%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 43,777
Percentage of debt securities by credit rating agencies	4.20%
Investment Grade | Foreign government bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 144,010
Percentage of debt securities by credit rating agencies	91.00%
Investment Grade | Foreign government bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 11,761
Percentage of debt securities by credit rating agencies	7.40%
Investment Grade | Governmental agency bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 299,095
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | Governmental agency mortgage-backed securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 3,348,376
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | U.S. corporate debt securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 250,839
Percentage of debt securities by credit rating agencies	46.50%
Investment Grade | U.S. corporate debt securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 218,235
Percentage of debt securities by credit rating agencies	40.50%
Investment Grade | Foreign corporate debt securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 129,565
Percentage of debt securities by credit rating agencies	40.10%
Investment Grade | Foreign corporate debt securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 167,265
Percentage of debt securities by credit rating agencies	51.80%
Investment Grade | Preferred stocks | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 28
Percentage of equity securities by credit rating agencies	0.20%
Investment Grade | Preferred stocks | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 12,589
Percentage of equity securities by credit rating agencies	94.60%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 100,072
Estimated fair value, Total	$ 100,763
Percentage of debt securities by credit rating agencies	1.70%
Percentage of investment portfolio by credit rating agencies	1.70%
Non-Investment Grade | Municipal bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,338
Percentage of debt securities by credit rating agencies	0.10%
Non-Investment Grade | Foreign government bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 2,527
Percentage of debt securities by credit rating agencies	1.60%
Non-Investment Grade | U.S. corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 70,207
Percentage of debt securities by credit rating agencies	13.00%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 26,000
Percentage of debt securities by credit rating agencies	8.10%
Non-Investment Grade | Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 691
Percentage of equity securities by credit rating agencies	5.20%