Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 5,529,860	$ 4,752,684
Equity securities, estimated fair value	447,749	466,516
Debt and Equity securities, estimated fair value	5,977,609	5,219,200
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Equity securities, estimated fair value	447,749	466,516
Debt and Equity securities, estimated fair value	447,749	466,516
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,529,860	4,708,150
Equity securities, estimated fair value	0	0
Debt and Equity securities, estimated fair value	5,529,860	4,708,150
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	44,534
Equity securities, estimated fair value	0	0
Debt and Equity securities, estimated fair value	0	44,534
U.S. Treasury Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	199,956	173,998
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	199,956	173,998
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	1,074,188	1,035,937
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	1,074,188	1,035,937
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	154,002	169,488
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	154,002	169,488
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	326,549	211,470
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	326,549	211,470
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	2,738,770	2,158,957
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	2,738,770	2,158,957
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	763,805	743,215
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	763,805	700,347
U.S. Corporate Debt Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	42,868
Foreign Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	272,590	259,619
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	272,590	257,953
Foreign Corporate Debt Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	1,666
Preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	18,432	18,990
Preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	18,432	18,990
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	429,317	447,526
Common Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	429,317	447,526
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