Debt and Equity Securities (Composition of Investment Portfolio by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 5,959,345	$ 5,713,811
Equity securities, Estimated fair value	344,790	353,535
Estimated fair value, Total	$ 5,973,233
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	$ 5,369,986
Estimated fair value, Total	$ 5,370,035
Percentage of debt securities by credit rating agencies	90.10%
Percentage of investment portfolio by credit rating agencies	89.90%
Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 398,213
Estimated fair value, Total	$ 410,842
Percentage of debt securities by credit rating agencies	6.70%
Percentage of investment portfolio by credit rating agencies	6.90%
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 191,146
Estimated fair value, Total	$ 192,356
Percentage of debt securities by credit rating agencies	3.20%
Percentage of investment portfolio by credit rating agencies	3.20%
U.S. Treasury Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 149,814	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	$ 149,814
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	$ 1,078,419	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	$ 1,028,992
Percentage of debt securities by credit rating agencies	95.40%
Municipal Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 43,349
Percentage of debt securities by credit rating agencies	4.00%
Municipal Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 6,078
Percentage of debt securities by credit rating agencies	0.60%
Foreign Government Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 153,352	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	$ 132,570
Percentage of debt securities by credit rating agencies	86.50%
Foreign Government Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 17,369
Percentage of debt securities by credit rating agencies	11.30%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 3,413
Percentage of debt securities by credit rating agencies	2.20%
Governmental Agency Bonds
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 352,555	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	$ 352,555
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	$ 3,335,282	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	$ 3,335,282
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	$ 599,835	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	$ 249,623
Percentage of debt securities by credit rating agencies	41.60%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 196,564
Percentage of debt securities by credit rating agencies	32.80%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 153,648
Percentage of debt securities by credit rating agencies	25.60%
Foreign Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 290,088	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	$ 121,150
Percentage of debt securities by credit rating agencies	41.70%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 140,931
Percentage of debt securities by credit rating agencies	48.60%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	$ 28,007
Percentage of debt securities by credit rating agencies	9.70%
Preferred stocks
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 13,888	$ 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	$ 49
Percentage of equity securities by credit rating agencies	0.40%
Preferred stocks | Investment Grade | BBB+ to BBB-
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 12,629
Percentage of equity securities by credit rating agencies	90.90%
Preferred stocks | Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Equity securities, Estimated fair value	$ 1,210
Percentage of equity securities by credit rating agencies	8.70%