Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Issuance of common stock through performance stock awards			286,350			150,259
Exercise of stock options	28,152	85,379	186,521	125,694	132,898	54,480
Issuance of common stock through employee purchase plan	26,726	31,455	44,161	28,877	28,864	39,514
Common stock shares issued for cash	266,410				213,276
Purchase of treasury stock			111,835			57,619
Common stock dividends/distributions	$ 0.96	$ 0.96	$ 0.96	$ 0.91	$ 0.91	$ 0.91
Preferred stock dividend shares	$ 15.95	$ 15.95	$ 15.95	$ 15.95	$ 15.95	$ 15.95
Issuance of common stock for purchase of assets				163,137
Additional Paid-in Capital [Member]
Issuance of common stock through performance stock awards			286,350			150,259
Exercise of stock options	28,152	85,379	186,521	125,694	132,898	54,480
Issuance of common stock through employee purchase plan	26,726	31,455	44,161	28,877	28,864	39,514
Issuance of common stock for purchase of assets				163,137
Treasury Stock [Member]
Purchase of treasury stock			111,835			57,619
Accumulated Deficit [Member]
Common stock dividends/distributions	$ 0.96	$ 0.96	$ 0.96	$ 0.91	$ 0.91	$ 0.91
Preferred stock dividend shares	$ 15.95	$ 15.95	$ 15.95	$ 15.95	$ 15.95	$ 15.95