Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,119,843	$ 4,752,684
Equity securities, Estimated fair value	438,695	466,516
Estimated fair value, Total	$ 5,137,789
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	$ 169,720	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	$ 1,094,435	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	$ 145,920	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	$ 257,020	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	$ 2,439,250	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	$ 748,924	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	$ 264,574	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	$ 17,946	$ 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	$ 4,419,365
Estimated fair value, Total	$ 4,419,423
Percentage of debt securities by credit rating agencies	86.30%
Percentage of investment portfolio by credit rating agencies	86.00%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 439,583
Estimated fair value, Total	$ 452,689
Percentage of debt securities by credit rating agencies	8.60%
Percentage of investment portfolio by credit rating agencies	8.80%
Investment Grade | U.S. Treasury Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 169,720
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	$ 1,011,354
Percentage of debt securities by credit rating agencies	92.40%
Investment Grade | Municipal Bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 59,805
Percentage of debt securities by credit rating agencies	5.50%
Investment Grade | Foreign Government Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 117,244
Percentage of debt securities by credit rating agencies	80.30%
Investment Grade | Foreign Government Bonds | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 23,596
Percentage of debt securities by credit rating agencies	16.20%
Investment Grade | Governmental Agency Bonds | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 257,020
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	$ 2,439,250
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	$ 301,840
Percentage of debt securities by credit rating agencies	40.30%
Investment Grade | U.S. Corporate Debt Securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 245,345
Percentage of debt securities by credit rating agencies	32.80%
Investment Grade | Foreign Corporate Debt Securities | A- or Higher
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 122,937
Percentage of debt securities by credit rating agencies	46.50%
Investment Grade | Foreign Corporate Debt Securities | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 110,837
Percentage of debt securities by credit rating agencies	41.90%
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	$ 13,106
Percentage of equity securities by credit rating agencies	73.00%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 260,895
Estimated fair value, Total	$ 265,677
Percentage of debt securities by credit rating agencies	5.10%
Percentage of investment portfolio by credit rating agencies	5.20%
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	$ 23,276
Percentage of debt securities by credit rating agencies	2.10%
Non-Investment Grade | Foreign Government Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,080
Percentage of debt securities by credit rating agencies	3.50%
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	$ 201,739
Percentage of debt securities by credit rating agencies	26.90%
Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 30,800
Percentage of debt securities by credit rating agencies	11.60%
Non-Investment Grade | Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 4,782
Percentage of equity securities by credit rating agencies	26.70%