Investments in debt and equity securities available-for-sale - Amortized Costs and Fair Values (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	$ 575,396
Equity securities, Amortized costs	28,046	$ 27,497
Total, Amortized costs	603,442	565,384
Debt securities, Fair values	601,762
Equity securities, Fair values	34,286	31,253
Total, Fair values	636,048	579,849
Municipal
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	70,253	70,300
Debt securities, Fair values	73,492	72,008
Corporate
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	334,085	303,870
Debt securities, Fair values	351,069	309,461
Foreign
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	158,265	149,914
Debt securities, Fair values	164,255	153,221
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized costs	12,793	13,803
Debt securities, Fair values	$ 12,946	$ 13,906