Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	$ 4,600,632	$ 3,852,664
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,279,347	3,450,252
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	321,285	402,412
U.S. Treasury Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	129,372	64,982
Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	703,795	587,677
Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	130,101	134,938
Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	418,091	197,874
Governmental Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	2,055,673	1,812,162
Non-Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value		16,538
U.S. Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	634,683	452,129
Foreign Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	207,632	183,952
Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,467	15,525
Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	305,818	386,887
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	321,285	402,412
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	321,285	402,412
Level 1 | Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	15,467	15,525
Level 1 | Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	305,818	386,887
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	4,229,208	3,433,714
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,229,208	3,433,714
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	129,372	64,982
Level 2 | Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	703,795	587,677
Level 2 | Foreign Government Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	130,101	134,938
Level 2 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	418,091	197,874
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	2,055,673	1,812,162
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	591,116	452,129
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	201,060	183,952
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	50,139	16,538
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	50,139	16,538
Level 3 | Non-Agency Mortgage-Backed Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value		$ 16,538
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	43,567
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	$ 6,572