STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - $ / shares	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Class A common stock
Dividends declared (in dollars per share)	$ 0.90	$ 0.90	$ 0.84	$ 0.84	$ 1.80	$ 1.68
Class B common stock
Dividends declared (in dollars per share)	$ 135.00	$ 135.00	$ 126.00	$ 126.00	$ 270.00	$ 252.00