Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,532,202	$ 615,839
Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 4,250,933
Percentage of debt securities in unrealized loss position	93.70%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 188,297
Percentage of debt securities in unrealized loss position	4.20%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 92,972
Percentage of debt securities in unrealized loss position	2.10%
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 59,445	7,744
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	$ 59,445
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	$ 576,749	74,045
Municipal bonds | Investment Grade | A- or higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 563,613
Percentage of debt securities in unrealized loss position	97.70%
Municipal bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 13,136
Percentage of debt securities in unrealized loss position	2.30%
Municipal 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%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 154,950	67,094
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	$ 153,987
Percentage of debt securities in unrealized loss position	99.40%
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	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Foreign government bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 963
Percentage of debt securities in unrealized loss position	0.60%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 75,547	15,353
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	$ 75,547
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	$ 3,070,159	388,420
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	$ 3,070,159
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	$ 432,702	43,865
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	$ 233,772
Percentage of debt securities in unrealized loss position	54.00%
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	$ 127,998
Percentage of debt securities in unrealized loss position	29.60%
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	$ 70,932
Percentage of debt securities in unrealized loss position	16.40%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 162,650	$ 19,318
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	$ 94,410
Percentage of debt securities in unrealized loss position	58.00%
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	$ 47,163
Percentage of debt securities in unrealized loss position	29.00%
Foreign corporate debt securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 21,077
Percentage of debt securities in unrealized loss position	13.00%