Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Sep. 30, 2021		Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 9,095,806		$ 6,354,822
Equity securities, estimated fair value	760,758		464,126
Debt and Equity securities, estimated fair value	9,856,564		6,818,948
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	760,758		464,126
Debt and Equity securities, estimated fair value	760,758		464,126
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	9,095,806		6,354,822
Debt and Equity securities, estimated fair value	9,095,806		6,354,822
U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	171,743		80,846
U.S. Treasury bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	171,743		80,846
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,613,801		1,248,808
Municipal bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,613,801		1,248,808
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	231,375		199,530
Foreign government bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	231,375		199,530
Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	211,507		263,922
Governmental agency bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	211,507		263,922
Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,373,158		3,474,618
Governmental agency mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,373,158		3,474,618
U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,022,389		680,697
U.S. corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,022,389		680,697
Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	471,833		406,401
Foreign corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	471,833		406,401
Preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	16,944		19,479
Preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	16,944		19,479
Common stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	743,814	[1]	444,647
Common stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	$ 743,814		$ 444,647

[1] Includes Offerpad Solutions Inc. with a cost of $85.0 million and estimated fair value of $280.3 million.  See below for further discussion about the Company’s investment in Offerpad Solutions Inc.