Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,999,922	$ 5,713,811
Equity securities, Estimated fair value	399,048	353,535
Estimated fair value, Total	$ 6,013,362
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	$ 5,485,113
Estimated fair value, Total	$ 5,485,160
Percentage of debt securities by credit rating agencies	91.40%
Percentage of investment portfolio by credit rating agencies	91.20%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 396,342
Estimated fair value, Total	$ 408,572
Percentage of debt securities by credit rating agencies	6.60%
Percentage of investment portfolio by credit rating agencies	6.80%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 118,467
Estimated fair value, Total	$ 119,630
Percentage of debt securities by credit rating agencies	2.00%
Percentage of investment portfolio by credit rating agencies	2.00%
U.S. Treasury Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 245,282	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	$ 245,282
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	$ 1,086,531	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	$ 1,045,937
Percentage of debt securities by credit rating agencies	96.30%
Municipal Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 39,214
Percentage of debt securities by credit rating agencies	3.60%
Municipal Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 1,380
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	$ 152,554	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	$ 133,639
Percentage of debt securities by credit rating agencies	87.60%
Foreign Government Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 13,453
Percentage of debt securities by credit rating agencies	8.80%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,462
Percentage of debt securities by credit rating agencies	3.60%
Governmental Agency Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 397,164	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	$ 397,164
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	$ 3,292,295	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	$ 3,292,295
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	$ 540,983	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	$ 250,349
Percentage of debt securities by credit rating agencies	46.30%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 201,802
Percentage of debt securities by credit rating agencies	37.30%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 88,832
Percentage of debt securities by credit rating agencies	16.40%
Foreign Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 285,113	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	$ 120,447
Percentage of debt securities by credit rating agencies	42.20%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 141,873
Percentage of debt securities by credit rating agencies	49.80%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 22,793
Percentage of debt securities by credit rating agencies	8.00%
Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 13,440	$ 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	$ 47
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	$ 12,230
Percentage of equity securities by credit rating agencies	91.00%
Preferred stocks | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 1,163
Percentage of equity securities by credit rating agencies	8.70%