Schedule of Parent Company Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ASSETS
Goodwill on purchased affiliates	$ 228,346	$ 209,758	$ 209,758
Securities available for sale and other	7,568,870	7,285,667
Total assets	19,094,245	17,500,960
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	86,070	8,810
Total liabilities	17,200,551	15,857,202
Shareholders' equity	1,893,694	1,643,758	$ 1,506,065	$ 1,279,345
Total liabilities and shareholders' equity	19,094,245	17,500,960
Parent Company [Member]
ASSETS
Investment in subsidiaries: Banks	1,596,292	1,312,575
Investment in subsidiaries: Non-banks	214,181	187,329
Total investment in subsidiaries	1,810,473	1,499,904
Goodwill on purchased affiliates	5,011	5,011
Cash	74,432	80,958
Securities available for sale and other	79,635	65,628
Total assets	1,969,551	1,651,501
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt	66,158
Accrued expenses and other	9,699	7,743
Total liabilities	75,857	7,743
Shareholders' equity	1,893,694	1,643,758
Total liabilities and shareholders' equity	$ 1,969,551	$ 1,651,501