Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 5,999,922	$ 5,713,811
Equity securities, estimated fair value	399,048	353,535
Debt and Equity securities, estimated fair value	6,398,970	6,067,346
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	0	0
Equity securities, estimated fair value	399,048	353,535
Debt and Equity securities, estimated fair value	399,048	353,535
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,999,922	5,713,811
Equity securities, estimated fair value	0	0
Debt and Equity securities, estimated fair value	5,999,922	5,713,811
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	245,282	162,506
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	245,282	162,506
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	1,086,531	1,045,035
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	1,086,531	1,045,035
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	152,554	157,297
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	152,554	157,297
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	397,164	316,167
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	397,164	316,167
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	3,292,295	3,202,599
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	3,292,295	3,202,599
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	540,983	561,260
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	540,983	561,260
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	285,113	268,947
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	285,113	268,947
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	13,440	14,162
Preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	13,440	14,162
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	385,608	339,373
Common Stock | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	385,608	339,373
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