Consolidated Statements of Changes in Shareholders' Equity (USD $)	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2011	$ 13,364,000	$ 266,026,000	$ 932,526,000	$ 3,121,000	$ 1,215,037,000
Balance (in shares) at Dec. 31, 2011	64,142,498
Net income per consolidated statements of income	0	0	117,283,000	0	117,283,000
Other comprehensive income (loss)	0	0	0	274,000	274,000
Cash dividends paid on common stock	0	0	(59,961,000)	0	(59,961,000)
Common stock issued, long-term incentive plan (in shares)	167,715
Common stock issued, long-term incentive plan	36,000	4,012,000	(5,285,000)	0	(1,237,000)
Common stock issued, business combination (in shares)	510,201
Common stock issued, business combination	106,000	11,894,000	0	0	12,000,000
Compensation expense, long-term incentive plan	0	3,973,000	0	0	3,973,000
Balance at Dec. 31, 2012	13,506,000	285,905,000	984,563,000	3,395,000	1,287,369,000
Balance (in shares) at Dec. 31, 2012	64,820,414
Net income per consolidated statements of income	0	0	117,060,000	0	117,060,000
Other comprehensive income (loss)	0	0	0	(47,126,000)	(47,126,000)
Cash dividends paid on common stock	0	0	(62,276,000)	0	(62,276,000)
Common stock issued, long-term incentive plan (in shares)	307,696
Common stock issued, long-term incentive plan	64,000	7,149,000	(4,381,000)	0	2,832,000
Common stock issued, business combination (in shares)	2,244,870
Common stock issued, business combination	468,000	53,027,000	0	0	53,495,000
Compensation expense, long-term incentive plan	0	3,599,000	0	0	3,599,000
Balance at Dec. 31, 2013	14,038,000	349,680,000	1,034,966,000	(43,731,000)	1,354,953,000
Balance (in shares) at Dec. 31, 2013	67,372,980				67,372,980
Net income per consolidated statements of income	0	0	123,562,000	0	123,562,000
Other comprehensive income (loss)	0	0	0	1,247,000	1,247,000
Cash dividends paid on common stock	0	0	(62,474,000)	0	(62,474,000)
Common stock issued, long-term incentive plan (in shares)	109,012
Common stock issued, long-term incentive plan	22,000	2,412,000	(3,934,000)	0	(1,500,000)
Compensation expense, long-term incentive plan	0	4,152,000	0	0	4,152,000
Balance at Dec. 31, 2014	$ 14,060,000	$ 356,244,000	$ 1,092,120,000	$ (42,484,000)	$ 1,419,940,000
Balance (in shares) at Dec. 31, 2014	67,481,992				67,481,992