Debt and Equity Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,412,496	$ 2,643,947
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 3,959,634
Percentage of debt securities in unrealized loss position	89.70%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 340,030
Percentage of debt securities in unrealized loss position	7.70%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 112,832
Percentage of debt securities in unrealized loss position	2.60%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 187,236	116,103
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 187,236
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
U.S. Treasury Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 805,159	506,187
Percentage of debt securities in unrealized loss position	100.00%
Municipal Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 757,128
Percentage of debt securities in unrealized loss position	94.00%
Municipal Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 34,436
Percentage of debt securities in unrealized loss position	4.30%
Municipal Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 13,595
Percentage of debt securities in unrealized loss position	1.70%
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 134,290	105,620
Percentage of debt securities in unrealized loss position	100.00%
Foreign Government Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 106,665
Percentage of debt securities in unrealized loss position	79.50%
Foreign Government Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 22,758
Percentage of debt securities in unrealized loss position	16.90%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,867
Percentage of debt securities in unrealized loss position	3.60%
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 320,019	149,444
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 320,019
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental Agency Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,245,204	1,446,830
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,245,204
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Governmental Agency Mortgage-Backed Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 528,301	232,541
Percentage of debt securities in unrealized loss position	100.00%
U.S. Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 260,752
Percentage of debt securities in unrealized loss position	49.30%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 193,826
Percentage of debt securities in unrealized loss position	36.70%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 73,723
Percentage of debt securities in unrealized loss position	14.00%
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 192,287	$ 87,222
Percentage of debt securities in unrealized loss position	100.00%
Foreign Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 82,630
Percentage of debt securities in unrealized loss position	43.00%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 89,010
Percentage of debt securities in unrealized loss position	46.30%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 20,647
Percentage of debt securities in unrealized loss position	10.70%