Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	$ 4,957,448	$ 4,600,632
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,553,363	4,279,347
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	404,085	321,285
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	404,085	321,285
Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	404,085	321,285
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,500,430	4,229,208
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,500,430	4,229,208
Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	52,933	50,139
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	52,933	50,139
Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	151,391	129,372
U.S. Treasury Bonds | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
U.S. Treasury Bonds | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	151,391	129,372
U.S. Treasury Bonds | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	984,333	703,795
Municipal Bonds | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Municipal Bonds | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	984,333	703,795
Municipal Bonds | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	140,048	130,101
Foreign Government Bonds | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Foreign Government Bonds | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	140,048	130,101
Foreign Government Bonds | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	193,868	418,091
Governmental Agency Bonds | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Governmental Agency Bonds | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	193,868	418,091
Governmental Agency Bonds | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,163,673	2,055,673
Governmental Agency Mortgage-Backed Securities | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Governmental Agency Mortgage-Backed Securities | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,163,673	2,055,673
Governmental Agency Mortgage-Backed Securities | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	678,524	634,683
U.S. Corporate Debt Securities | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
U.S. Corporate Debt Securities | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	631,859	591,116
U.S. Corporate Debt Securities | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	46,665	43,567
Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	241,526	207,632
Foreign Corporate Debt Securities | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Foreign Corporate Debt Securities | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	235,258	201,060
Foreign Corporate Debt Securities | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	6,268	6,572
Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,582	15,467
Preferred stocks | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,582	15,467
Preferred stocks | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Preferred stocks | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	388,503	305,818
Common Stock | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	388,503	305,818
Common Stock | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Common Stock | Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	$ 0	$ 0