Consolidated Statements of Common Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Balance at the beginning of the period at Dec. 31, 2012	$ 473,712	$ 419	$ 221,013	$ 252,280
Balance (in shares) at Dec. 31, 2012		41,908
Increase (Decrease) in Stockholders' Equity
Net income	47,254			47,254
Issuance of common stock	107,409	$ 58	107,351
Issuance of common stock (in shares)		5,833
Dividends paid on common stock ($0.670, $0.650 and $0.640 per share for the years ended December 31, 2015, 2014 and 2013 respectively)	(29,619)			(29,619)
Balance at the end of the period at Dec. 31, 2013	598,756	$ 477	328,364	269,915
Balance (in shares) at Dec. 31, 2013		47,741
Increase (Decrease) in Stockholders' Equity
Net income	56,738			56,738
Issuance of common stock	2,195	$ 1	2,194
Issuance of common stock (in shares)		65
Dividends paid on common stock ($0.670, $0.650 and $0.640 per share for the years ended December 31, 2015, 2014 and 2013 respectively)	(31,063)			(31,063)
Balance at the end of the period at Dec. 31, 2014	626,626	$ 478	330,558	295,590
Balance (in shares) at Dec. 31, 2014		47,806
Increase (Decrease) in Stockholders' Equity
Net income	45,017			45,017
Issuance of common stock	2,578	$ 1	2,577
Issuance of common stock (in shares)		69
Dividends paid on common stock ($0.670, $0.650 and $0.640 per share for the years ended December 31, 2015, 2014 and 2013 respectively)	(32,066)			(32,066)
Balance at the end of the period at Dec. 31, 2015	$ 642,155	$ 479	$ 333,135	$ 308,541
Balance (in shares) at Dec. 31, 2015		47,875