Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	$ 5,249,669	$ 4,957,448
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,803,484	4,553,363
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	446,185	404,085
U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	145,453	151,391
Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	1,072,362	984,333
Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	161,649	140,048
Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	221,718	193,868
Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,225,920	2,163,673
U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	726,928	678,524
Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	249,454	241,526
Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	18,527	15,582
Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	427,658	388,503
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	446,185	404,085
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	446,185	404,085
Level 1 | U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 1 | Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	18,527	15,582
Level 1 | Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	427,658	388,503
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,790,102	4,500,430
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,790,102	4,500,430
Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 2 | U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	145,453	151,391
Level 2 | Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	1,072,362	984,333
Level 2 | Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	161,649	140,048
Level 2 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	221,718	193,868
Level 2 | Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,225,920	2,163,673
Level 2 | U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	714,419	631,859
Level 2 | Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	248,581	235,258
Level 2 | Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 2 | Common Stock | 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	13,382	52,933
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	13,382	52,933
Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	12,509	46,665
Level 3 | Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	873	6,268
Level 3 | Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	0	0
Level 3 | Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	$ 0	$ 0