Parent Company Condensed Financial Information - Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Information [Line Items]
Net Income	$ 57,910	$ 50,539	$ 34,200
Tax (benefit) expense from stock-based compensation	300	200	300
Net Cash Provided by Operating Activities	78,103	86,076	83,890
Net Cash Used in Investing Activities	(437,048)	(309,683)	(8,323)
(Purchase) Sale of treasury shares, net			1,047
Cash dividends paid to common shareholders	(20,203)	(18,137)	(17,357)
Tax benefit (expense) from stock-based compensation	16	96	(30)
Net Cash Provided by (Used in) Financing Activities	360,169	(5,748)	(8,382)
Net increase (decrease) in cash	1,224	(229,355)	67,185
Cash and cash equivalents at beginning of year	108,356	337,711	270,526
Cash and Cash Equivalents at End of Year	109,580	108,356	337,711
Parent Company [Member]
Condensed Financial Information [Line Items]
Net Income	57,910	50,539	34,200
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions	(14,250)	(29,785)	(1,188)
Tax (benefit) expense from stock-based compensation	(16)	(96)	30
Other	(106)	121	1,023
Net Cash Provided by Operating Activities	43,538	20,779	34,065
Net investments in subsidiaries	0	0	(5,035)
Acquisitions	0	0	(14,123)
Net Cash Used in Investing Activities	0	0	(19,158)
(Purchase) Sale of treasury shares, net	(163)	(88)	998
Cash dividends paid to common shareholders	(20,215)	(18,137)	(17,357)
Tax benefit (expense) from stock-based compensation	16	96	(30)
Net Cash Provided by (Used in) Financing Activities	(20,362)	(18,129)	(16,389)
Net increase (decrease) in cash	23,176	2,650	(1,482)
Cash and cash equivalents at beginning of year	14,852	12,202	13,684
Cash and Cash Equivalents at End of Year	$ 38,028	$ 14,852	$ 12,202