Fair Value Measurements (Fair Value of Assets Measured on Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	$ 4,326,145	$ 3,852,664
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,039,816	3,450,252
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	286,329	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	97,059	64,982
Municipal Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	624,406	587,677
Foreign Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	181,021	196,750
Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	359,837	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	2,046,631	1,812,162
Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	730,862	574,269
Preferred stocks | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	18,028	15,525
Common Stock | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	268,301	386,887
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 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	286,329	402,412
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	286,329	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	18,028	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	268,301	386,887
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	4,007,250	3,433,714
Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	4,007,250	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	97,059	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	624,406	587,677
Level 2 | Foreign Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	181,021	196,750
Level 2 | Governmental Agency Bonds | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	359,837	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	2,046,631	1,812,162
Level 2 | Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	698,296	574,269
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Total assets	32,566	16,538
Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	32,566	16,538
Level 3 | Corporate Debt Securities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Available for sale securities, estimated fair value	32,566
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