Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,529,860	$ 4,752,684
Equity securities, Estimated fair value	447,749	466,516
Estimated fair value, Total	$ 5,548,292
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	$ 199,956	173,998
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	$ 1,074,188	1,035,937
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	$ 154,002	169,488
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	$ 326,549	211,470
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	$ 2,738,770	2,158,957
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	$ 763,805	743,215
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	$ 272,590	259,619
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	$ 18,432	$ 18,990
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	$ 4,814,659
Estimated fair value, Total	$ 4,814,717
Percentage of debt securities by credit rating agencies	87.10%
Percentage of investment portfolio by credit rating agencies	86.80%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 437,285
Estimated fair value, Total	$ 453,302
Percentage of debt securities by credit rating agencies	7.90%
Percentage of investment portfolio by credit rating agencies	8.20%
Investment Grade | U.S. Treasury Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 199,956
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | U.S. Treasury Bonds | 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%
Investment Grade | Municipal Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 997,639
Percentage of debt securities by credit rating agencies	92.90%
Investment Grade | Municipal Bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 52,149
Percentage of debt securities by credit rating agencies	4.90%
Investment Grade | Foreign Government Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 125,378
Percentage of debt securities by credit rating agencies	81.40%
Investment Grade | Foreign Government Bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 23,757
Percentage of debt securities by credit rating agencies	15.40%
Investment Grade | Governmental Agency Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 326,549
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | Governmental Agency Bonds | 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%
Investment Grade | Governmental Agency Mortgage-Backed Securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 2,738,770
Percentage of debt securities by credit rating agencies	100.00%
Investment Grade | Governmental Agency Mortgage-Backed Securities | 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%
Investment Grade | U.S. Corporate Debt Securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 303,068
Percentage of debt securities by credit rating agencies	39.70%
Investment Grade | U.S. Corporate Debt Securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 248,671
Percentage of debt securities by credit rating agencies	32.60%
Investment Grade | Foreign Corporate Debt Securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 123,299
Percentage of debt securities by credit rating agencies	45.20%
Investment Grade | Foreign Corporate Debt Securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 112,708
Percentage of debt securities by credit rating agencies	41.30%
Investment Grade | Preferred stocks | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 58
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	$ 16,017
Percentage of equity securities by credit rating agencies	86.90%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 277,916
Estimated fair value, Total	$ 280,273
Percentage of debt securities by credit rating agencies	5.00%
Percentage of investment portfolio by credit rating agencies	5.00%
Non-Investment Grade | U.S. Treasury Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Non-Investment Grade | Municipal Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 24,400
Percentage of debt securities by credit rating agencies	2.20%
Non-Investment Grade | Foreign Government Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 4,867
Percentage of debt securities by credit rating agencies	3.20%
Non-Investment Grade | Governmental Agency Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Non-Investment Grade | Governmental Agency Mortgage-Backed Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 0
Percentage of debt securities by credit rating agencies	0.00%
Non-Investment Grade | U.S. Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 212,066
Percentage of debt securities by credit rating agencies	27.70%
Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 36,583
Percentage of debt securities by credit rating agencies	13.50%
Non-Investment Grade | Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 2,357
Percentage of equity securities by credit rating agencies	12.80%