Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 7,211,498	$ 6,354,822
Equity securities, Estimated fair value	419,606	464,126
Estimated fair value, Total	$ 7,232,418
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	$ 156,117	80,846
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	$ 1,453,618	1,248,808
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	$ 205,292	199,530
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	$ 303,505	263,922
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	$ 4,015,850	3,474,618
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	$ 660,334	680,697
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	$ 416,782	406,401
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	$ 20,920	$ 19,479
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	$ 6,466,294
Estimated fair value, Total	$ 6,466,353
Percentage of debt securities by credit rating agencies	89.70%
Percentage of investment portfolio by credit rating agencies	89.40%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 630,463
Estimated fair value, Total	$ 649,831
Percentage of debt securities by credit rating agencies	8.70%
Percentage of investment portfolio by credit rating agencies	9.00%
Investment Grade | U.S. Treasury bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 156,117
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	$ 1,406,205
Percentage of debt securities by credit rating agencies	96.70%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 44,833
Percentage of debt securities by credit rating agencies	3.10%
Investment Grade | Foreign government bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 190,399
Percentage of debt securities by credit rating agencies	92.70%
Investment Grade | Foreign government bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 12,069
Percentage of debt securities by credit rating agencies	5.90%
Investment Grade | Governmental agency bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 303,505
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	$ 4,015,850
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	$ 246,251
Percentage of debt securities by credit rating agencies	37.30%
Investment Grade | U.S. corporate debt securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 336,566
Percentage of debt securities by credit rating agencies	51.00%
Investment Grade | Foreign corporate debt securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 147,967
Percentage of debt securities by credit rating agencies	35.50%
Investment Grade | Foreign corporate debt securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 236,995
Percentage of debt securities by credit rating agencies	56.90%
Investment Grade | Preferred stocks | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 59
Percentage of equity securities by credit rating agencies	0.30%
Investment Grade | Preferred stocks | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 19,368
Percentage of equity securities by credit rating agencies	92.60%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 114,741
Estimated fair value, Total	$ 116,234
Percentage of debt securities by credit rating agencies	1.60%
Percentage of investment portfolio by credit rating agencies	1.60%
Non-Investment Grade | Municipal bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 2,580
Percentage of debt securities by credit rating agencies	0.20%
Non-Investment Grade | Foreign government bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 2,824
Percentage of debt securities by credit rating agencies	1.40%
Non-Investment Grade | U.S. corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 77,517
Percentage of debt securities by credit rating agencies	11.70%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 31,820
Percentage of debt securities by credit rating agencies	7.60%
Non-Investment Grade | Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 1,493
Percentage of equity securities by credit rating agencies	7.10%