Equity (Details) - USD ($)		1 Months Ended	3 Months Ended								12 Months Ended			194 Months Ended
	Jan. 15, 2019	Feb. 08, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	May 24, 2018	Dec. 31, 1995
Stock issued during period, shares, new issue, main offering													1,475,000
Shares issued during period, shares, new issues, price per share													$ 116.67
Stock issued during period, shares, new issues, additional offering													221,250
Issuance of common shares, shares													1,696,250
Stock issued during period, shares, new issues, net proceeds													$ 197,200,000
Stock repurchase program, number of shares authorized to be repurchased, percent															10.00%	10.00%
Treasury stock, shares, acquired														39,618,299
Treasury stock, value, acquired, cost method														$ 1,600,000,000
Treasury stock acquired, average cost per share														$ 41.57
Treasury stock, shares, retired														33,475,406
Treasury stock, retired, cost method, amount													$ 0	$ 1,200,000,000
Treasury stock, shares			479,407				612,047				479,407	612,047		479,407
Stock repurchase program, remaining authorized repurchase amount (in shares)			4,000,218								4,000,218			4,000,218
Cash dividends per share (in dollar per share)			$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 0.55	$ 2.20	$ 2.20	$ 2.20
Subsequent Event [Member]
Treasury stock, shares, acquired		35
Treasury stock, value, acquired, cost method		$ 2,400
Cash dividends per share (in dollar per share)	$ 0.55
Dividends payable, date declared	Jan. 15, 2019
Dividends payable, date to be paid	Feb. 15, 2019
Dividends payable, date of record	Jan. 25, 2019