Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 7,211,498	$ 6,354,822
Equity securities, estimated fair value	419,606	464,126
Debt and Equity securities, estimated fair value	7,631,104	6,818,948
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Equity securities, estimated fair value	419,606	464,126
Debt and Equity securities, estimated fair value	419,606	464,126
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	7,211,498	6,354,822
Equity securities, estimated fair value	0	0
Debt and Equity securities, estimated fair value	7,211,498	6,354,822
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Equity securities, estimated fair value	0	0
Debt and Equity securities, estimated fair value	0	0
U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	156,117	80,846
U.S. Treasury bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
U.S. Treasury bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	156,117	80,846
U.S. Treasury bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,453,618	1,248,808
Municipal bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Municipal bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,453,618	1,248,808
Municipal bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	205,292	199,530
Foreign government bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Foreign government bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	205,292	199,530
Foreign government bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	303,505	263,922
Governmental agency bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Governmental agency bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	303,505	263,922
Governmental agency bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	4,015,850	3,474,618
Governmental agency mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Governmental agency mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	4,015,850	3,474,618
Governmental agency mortgage-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	660,334	680,697
U.S. corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
U.S. corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	660,334	680,697
U.S. corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	416,782	406,401
Foreign corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Foreign corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	416,782	406,401
Foreign corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	20,920	19,479
Preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	20,920	19,479
Preferred stocks | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	0	0
Preferred stocks | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	0	0
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	398,686	444,647
Common Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	398,686	444,647
Common Stock | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	0	0
Common Stock | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	$ 0	$ 0