Consolidated Statement of Changes in Shareholders' Equity (USD $) In Thousands	Common Stock [Member]	Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2009	$ 422,008	$ 14,859	$ 137,309	$ 0	$ 574,176
Balance (in shares) at Dec. 31, 2009	29,976,506
Net income			54,857		54,857
Cash dividends on common stock			(60,707)		(60,707)
Preferred stock dividends of subsidiary, noncontrolling interests			(3)		(3)
Issuance of common stock - no par value	505,486				505,486
Issuance of common stock - no par value (in shares)	20,528,945
Stock based compensation		2,167			2,167
Other comprehensive income (loss), net				166	166
Balance at Dec. 31, 2010	927,494	17,026	131,456	166	1,076,142
Balance (in shares) at Dec. 31, 2010	50,505,451
Net income			99,710		99,710
Cash dividends on common stock			(87,320)		(87,320)
Preferred stock dividends of subsidiary, noncontrolling interests			(54)		(54)
Issuance of common stock - no par value	3,659				3,659
Issuance of common stock - no par value (in shares)	140,039
Stock based compensation		2,765			2,765
Other comprehensive income (loss), net				(541)	(541)
Balance at Dec. 31, 2011	931,153	19,791	143,792	(375)	1,094,361
Balance (in shares) at Dec. 31, 2011	50,645,490
Net income			103,701		103,701
Cash dividends on common stock			(87,529)		(87,529)
Preferred stock dividends of subsidiary, noncontrolling interests			(64)		(64)
Issuance of common stock - no par value	5,549				5,549
Issuance of common stock - no par value (in shares)	229,093
Stock based compensation		609			609
Other comprehensive income (loss), net				(74)	(74)
Balance at Dec. 31, 2012	$ 936,702	$ 20,400	$ 159,900	$ (449)	$ 1,116,553
Balance (in shares) at Dec. 31, 2012	50,874,583