Debt and Equity Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,924,554	$ 615,839
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 109,133	7,744
Percentage of debt securities in unrealized loss position	100.00%
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 468,695	74,045
Percentage of debt securities in unrealized loss position	100.00%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 91,324	67,094
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 69,387	15,353
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,897,242	388,420
Percentage of debt securities in unrealized loss position	100.00%
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 171,995	43,865
Percentage of debt securities in unrealized loss position	100.00%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 116,778	$ 19,318
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	$ 2,727,752
Percentage of debt securities in unrealized loss position	93.30%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 141,819
Percentage of debt securities in unrealized loss position	4.80%
Investment Grade | U.S. Treasury bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 109,133
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | Municipal bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 462,004
Percentage of debt securities in unrealized loss position	98.60%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,691
Percentage of debt securities in unrealized loss position	1.40%
Investment Grade | Foreign government bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 89,161
Percentage of debt securities in unrealized loss position	97.60%
Investment Grade | Foreign government bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 612
Percentage of debt securities in unrealized loss position	0.70%
Investment Grade | Governmental agency bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 69,387
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | Governmental agency mortgage-backed securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,897,242
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | U.S. corporate debt securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 48,707
Percentage of debt securities in unrealized loss position	28.30%
Investment Grade | U.S. corporate debt securities | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 80,898
Percentage of debt securities in unrealized loss position	47.10%
Investment Grade | Foreign corporate debt securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 52,118
Percentage of debt securities in unrealized loss position	44.60%
Investment Grade | Foreign corporate debt securities | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 53,618
Percentage of debt securities in unrealized loss position	45.90%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 54,983
Percentage of debt securities in unrealized loss position	1.90%
Non-Investment Grade | Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,551
Percentage of debt securities in unrealized loss position	1.70%
Non-Investment Grade | U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 42,390
Percentage of debt securities in unrealized loss position	24.60%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 11,042
Percentage of debt securities in unrealized loss position	9.50%