Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,882,421	$ 5,713,811
Equity securities, Estimated fair value	416,488	353,535
Estimated fair value, Total	$ 5,896,683
Percentage of debt securities by credit rating agencies	100.00%
Percentage of investment portfolio by credit rating agencies	100.00%
Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,316,177
Estimated fair value, Total	$ 5,316,220
Percentage of debt securities by credit rating agencies	90.40%
Percentage of investment portfolio by credit rating agencies	90.20%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 442,452
Estimated fair value, Total	$ 455,576
Percentage of debt securities by credit rating agencies	7.50%
Percentage of investment portfolio by credit rating agencies	7.70%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 123,792
Estimated fair value, Total	$ 124,887
Percentage of debt securities by credit rating agencies	2.10%
Percentage of investment portfolio by credit rating agencies	2.10%
U.S. Treasury Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 209,574	162,506
Percentage of debt securities by credit rating agencies	100.00%
U.S. Treasury Bonds | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 209,574
Percentage of debt securities by credit rating agencies	100.00%
U.S. Treasury Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
U.S. Treasury Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Municipal Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,090,590	1,045,035
Percentage of debt securities by credit rating agencies	100.00%
Municipal Bonds | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,048,526
Percentage of debt securities by credit rating agencies	96.20%
Municipal Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 40,686
Percentage of debt securities by credit rating agencies	3.70%
Municipal Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,378
Percentage of debt securities by credit rating agencies	0.10%
Foreign Government Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 161,360	157,297
Percentage of debt securities by credit rating agencies	100.00%
Foreign Government Bonds | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 141,959
Percentage of debt securities by credit rating agencies	87.90%
Foreign Government Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 13,820
Percentage of debt securities by credit rating agencies	8.60%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,581
Percentage of debt securities by credit rating agencies	3.50%
Governmental Agency Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 373,436	316,167
Percentage of debt securities by credit rating agencies	100.00%
Governmental Agency Bonds | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 373,436
Percentage of debt securities by credit rating agencies	100.00%
Governmental Agency Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental Agency Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental Agency Mortgage-Backed Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 3,186,157	3,202,599
Percentage of debt securities by credit rating agencies	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 3,186,157
Percentage of debt securities by credit rating agencies	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Governmental Agency Mortgage-Backed Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
U.S. Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 582,604	561,260
Percentage of debt securities by credit rating agencies	100.00%
U.S. Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 251,999
Percentage of debt securities by credit rating agencies	43.20%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 236,341
Percentage of debt securities by credit rating agencies	40.60%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 94,264
Percentage of debt securities by credit rating agencies	16.20%
Foreign Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 278,700	268,947
Percentage of debt securities by credit rating agencies	100.00%
Foreign Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 104,526
Percentage of debt securities by credit rating agencies	37.50%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 151,605
Percentage of debt securities by credit rating agencies	54.40%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 22,569
Percentage of debt securities by credit rating agencies	8.10%
Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 14,262	$ 14,162
Percentage of equity securities by credit rating agencies	100.00%
Preferred stocks | Investment Grade | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 43
Percentage of equity securities by credit rating agencies	0.30%
Preferred stocks | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 13,124
Percentage of equity securities by credit rating agencies	92.00%
Preferred stocks | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 1,095
Percentage of equity securities by credit rating agencies	7.70%